SQUAMATA
The order Squamata, so called because of the dermal covering of
overlapping horny scales, comprises the great majority of living
reptiles. Although the scaly covering is characteristic of nearly all
the members of the order, the most essential differences distinguishing
them from other reptiles are, as usual, found in the skeleton, and
especially in the skull. The quadrate bone, that to which the lower
jaw is articulated on each side, is not wedged in immovably between
other bones of the skull, as in all other reptiles, but is, instead,
freely articulated with the cranium in such a way that its lower end
moves both backward and forward, as well as inward and outward. This
freedom of movement has in the past been thought to be due to the
loss of a lower temporal arch, a bony bar connecting the lower end
of the quadrate with the hind end of the upper jaw, which is very
characteristic, for instance, of the Rhynchocephalia. Indeed, because
of the many primitive characters which the lizards possess, it has
generally been supposed that the order was an early branch of the
rhynchocephalian stem. But we are now quite sure that the lizards
are as primitive as the Rhynchocephalia, and that their origin, as
an independent branch of the reptilian stem, goes quite as far if
not farther back—quite sure that the ancestors of the lizards never
had a lower temporal arcade and two temporal vacuities, but that
the looseness of the quadrate bone has been due to the gradual loss
of a bone which covered the whole side of the skull until only the
upper part of it was left. In other words, the ancestral skull of
the Squamata must have been like that of _Araeoscelis_, more fully
described under the Protorosauria, a group than which there is perhaps
none more closely allied to the Squamata.
The bones of the roof of the mouth of the Squamata—that is, of the
palate—are narrow and long, and are not closely articulated, as in
most other reptiles; they often bear teeth, a primitive character. The
teeth of all living lizards and snakes are not inserted in sockets,
as are those of the crocodiles, but are co-ossified to the margins or
sides of the jaws or the bones of the palate. But this is probably not
a primitive character; doubtless the teeth of the early lizards were
inserted in sockets like those of most other reptiles. The shoulder
bones are absent in many and vestigial in some others. When present
and fully developed, they comprise the shoulder-blades or scapulae,
a single coracoid on each side, the clavicles, and an interclavicle.
The vertebrae, except in some lizards, are procoelous, that is, with
the body concave in front and convex behind, a peculiar structure
that was developed only in crawling animals. In addition to the usual
articulations for the union of the vertebrae there are also, in some
of the lizards and mosasaurs and all of the snakes, additional ones
called the zygosphene and zygantrum, which will be best understood
by reference to Fig. 12, p. 28. But little less characteristic than
the loose articulation of the lower jaws, so unique in this order of
reptiles, is the manner of attachment of the ribs. They are always
single-headed, articulating only with the body or lower part of the
vertebra. The single-headed ribs of the plesiosaurs articulate with
a projection on each side of the arch of the vertebra; those of the
turtles to the space between the adjacent vertebrae; nearly all other
reptiles have double-headed ribs, articulating in various ways. This
character, it is seen, though apparently a simple one, immediately
distinguishes a lizard or a snake from all other animals, except the
thalattosaurs and protorosaurs.
There is much difference of opinion among naturalists as to the proper
classification of the different groups of this order of reptiles.
Usually it is divided into four suborders, the Lacertilia or lizards;
the Dolichosauria or long-necked lizards of the past; the Mosasauria,
or extinct swimming lizards; and the Serpentes or Ophidia, the snakes.
It matters very little which classification one accepts so long as it
is remembered that the first three groups are closely related to each
other.
LIZARDS
Popularly a lizard is any four-legged reptile covered with scales,
but such a definition is not strictly correct, since some lizards are
legless and some other four-legged reptiles are covered with horny
scales, notably the tuatera or _Sphenodon_ of New Zealand, a reptile
long classed with lizards, but now known to belong to quite a different
order. Bearing in mind those characters given as characters of the
order, it will be necessary to mention only those distinguishing the
lizards from the snakes.
[Illustration: FIG. 65.—Iguana. (By permission of the New York
Zoölogical Society)]
It is true that the great majority of lizards have four legs, while the
snakes are always functionally legless, but there are some lizards,
like the glass snakes and the amphisbaenas, or slow lizards, which
are quite legless and there are some snakes which have small but
functionless hind legs. As usual, more important differences are found
in the skull. The brain-case in all snakes is surrounded on all sides
by bone, for the better protection of the brain, with the head resting
quite prone on the ground. The brain of the lizards, for the most part,
is protected on the sides and in front by a simple membrane. Nearly all
lizards have movable eyelids, while snakes do not; snakes have a single
lung, and a protrusible tongue, which very few lizards possess; and
the lower jaws in front are united in the snakes by a ligament only.
Notwithstanding these differences, the snakes and lizards are closely
related animals, and must have come from a common ancestry; among all
reptiles the known geological history of the snakes is shortest.
Lizards, on the other hand, have a very high antiquity, beginning,
as we now know, at least as long ago as early Triassic times. They
still have many primitive characters in their structure and are the
least advanced type of reptiles now living, with the exception of the
tuatera. Their remains are seldom found in the rocks, probably because
they have always been so strictly terrestrial in habit, for the most
part seldom frequenting even the vicinity of the water. The true
lizards now living number about eighteen hundred kinds, classified into
about twenty families, divided among four chief groups, of which the
chameleons, the amphisbaenas, our common lizards, and the monitors are
representatives.
Most living lizards are inhabitants of warm climates, though some
extend rather far north in the temperate zone. With the exception of
New Zealand, and the polar and subpolar regions, lizards are found
in all parts of the world. The great majority live only in high and
dry places, though some are denizens of low and marshy places, a few
even not being averse to the water. They are, for the most part, spry
in their movements, some little ones scarcely six inches in length
taxing a vigorous man’s speed to capture; and many are expert climbers
of cliffs, trees, and even the ceilings of residences. Some, the
remarkable little flying dragons of Ceylon, have an extraordinary
development of the skin on the sides of the body, supported by the
expanded ribs, forming a sort of parachute whereby the creatures
can sail considerable distances through the air. Nearly all are
carnivorous, feeding upon small mammals, birds, other reptiles, frogs,
and insects; a few only are herbivorous, such as the iguanas, which are
often used for human food. Nearly all lizards are oviparous, laying
from two to thirty eggs. In size the great majority are small, less
than a foot in length; but some, such as the monitors and iguanas,
reach a length of from four to six feet, or even more, and certain
extinct monitors of India are known to have attained a length of thirty
feet. They are, for the most part, slender, graceful, prettily marked,
and quite inoffensive creatures. A few are short, flat, or stumpy in
shape, such as the so-called horned toad. One or two species only, the
“Gila monsters,” are reputed to be venomous.
[Illustration: FIG. 66.—_Amblyrhynchus cristatus_, the Galapagos
sea-lizard. (From Brehm)]
There is but a single species of lizard now living which is in any true
sense aquatic in habit, the well-known sea-lizard of the Galapagos
Islands, scientifically known as _Amblyrhynchus cristatus_. It is a
large lizard, with a short rounded head, a flat tail, and webbed feet.
Its specific name is derived from the erect fringed crest along its
back and tail. Its habits are best given in Darwin’s words:
It is extremely common on all the islands throughout the
group, and lives exclusively on the rocky sea-beaches,
being never found, at least I never saw one, even ten yards
inshore. It is a hideous looking creature, of a dirty black
color, stupid and sluggish in its movements. The usual
length of a full grown one is about a yard, but there are
some even four feet in length; a large one weighed twenty
pounds. The tails are flattened sideways, and all four
feet are partially webbed. They are occasionally seen some
hundred yards from the shore swimming about. When in the
water this lizard swims with perfect ease and quickness, by
a serpentine movement of the body and flattened tail—the
legs being motionless and closely collapsed to the sides.
A seaman on board sank one, with a heavy weight attached
to it, thinking thus to kill it directly; but when an
hour afterward he drew up the line it was quite active.
Their limbs and strong claws are admirably adapted for
crawling over the rugged and fissured masses of lava, which
everywhere forms the coast. The nature of this lizard’s food
(seaweed) as well as the structure of the tail and feet,
and the fact of its having been seen voluntarily swimming
out at sea, absolutely proves its aquatic habits; yet there
is in this respect one strange anomaly, namely, that when
frightened it will not leave the island. Hence it is easy
to drive these lizards down to any little point overhanging
the sea, where they will sooner allow a person to catch
hold of their tails than jump into the water. They do not
seem to have any notion of biting; but when much frightened
they squirt a drop of fluid from each nostril. I threw one
several times as far as I could into a deep pool left by
the retreating tide, but it invariably returned by a direct
line to the spot where I stood. It swam near the bottom,
with a very graceful and rapid movement, and occasionally
aided itself over the uneven ground with its feet. As soon
as it arrived near the edge, but still being under water,
it tried to conceal itself under the tufts of seaweed, or
it entered some crevice. I several times caught the same
lizard by driving it to a point, and, though possessed of
such perfect powers of diving and swimming, nothing could
induce it to enter the water; and as often as I threw it
in it returned in the manner described above. Perhaps this
singular piece of apparent stupidity may be accounted for by
the circumstance that this reptile has no enemies whatever
on shore, whereas at sea it must often fall a prey to the
numerous sharks.
These lizards are of much interest as indicating one of the ways in
which true land reptiles have become aquatic in their habits. Tempted
by the abundance of food growing in shallow water a little beyond their
reach, the reptiles ventured farther and farther to obtain it. The tail
gradually became a propelling organ, though the lizard still retained
in large measure its land habits and land feet, because of the dangers
from its water enemies. It is not at all improbable that, in course
of time, were these Galapagos lizards left unmolested, they would
become fleeter swimmers by the development of a terminal caudal fin
and paddle-like legs, thus competing with their aquatic enemies and no
longer needing recourse to the land for protection. They also serve to
indicate that long-tailed aquatic reptiles never used their legs to an
appreciable extent as organs of propulsion in the water.
[Illustration: FIG. 67.—_Varanus_, Australian monitor lizard. (By
permission of the New York Zoölogical Society.)]
_Flat-headed lizards._—Among the living lizards there is one group,
called the monitors, which have so many characters peculiar to
themselves that they seem rightfully entitled to an isolated place
among the lizards of the present time. The group includes about
thirty species, all belonging in the one genus _Varanus_, and all
living in India, Africa, and Australia. In size, some of the species
of _Varanus_ are the largest of all terrestrial lizards known in the
past or present; in other ways also they have reached the maximum of
specialization among lizards. The head is pointed, broad, and flat,
and the body and tail are long. They have nine vertebrae in the neck,
a larger number than is to be found in any other terrestrial lizard.
Unlike other lizards they have a protrusible tongue like that of the
snakes. All are carnivorous in habit, feeding upon small backboned
animals, insects, and especially upon eggs, which they crush between
their teeth while holding them aloft. Most species live wholly upon the
land, and some are arboreal. Others, especially those of the Nile, live
about water and are excellent swimmers. The terrestrial species have
a round tail and small external nostrils, but the water species have
the tail much flattened, and the nostrils have large cavities, which,
when closed under water, are said to serve as reservoirs of air for
respiration. Of one of these swimming species Annandale writes:
_Varanus salvator_ is common in Lower Siam where it is
equally at home on land, in water, and among the branches of
trees. The eggs are laid in hollow tree trunks. When in the
water the lizard swims beneath the surface, the legs being
closely applied to the sides, and the tail functioning both
as oar and rudder.
These lizards take to the water to escape from their land enemies and
not for food, a habit also known among certain other lizards, and
one precisely the reverse of that of the Galapagos lizards. It would
seem very probable that animals of such carnivorous habits as are the
monitors might easily learn to capture water animals for food and
thus eventually become aquatic in habit. This inclination toward, and
partial adaptation to, water habits in the monitors is of much interest
because in all probability the instinct is one of long inheritance from
those remote ancestors which gave origin to the truly aquatic members
of the order. Though the known geological history of the monitors does
not extend far back, they are so intimately allied in their anatomical
structure to the aquatic and semiaquatic lizards of Cretaceous times
that there could seem to be no doubt of the common ancestry.
_Dolichosaurs._—About fifty years ago Professor Owen, the famous
English paleontologist, described a peculiar semiaquatic lizard
from the Cretaceous rocks of England to which he gave the name
_Dolichosaurus_, in allusion to the slender form of the body. Just what
relations these slender lizards have to modern lizards has long been
a problem; some have thought that they were their progenitors, but
there are very good reasons for doubting this. No modern lizards, save
the monitors, have more than eight vertebrae in the neck, while these
dolichosaurs had as many as seventeen, a remarkable specialization for
aquatic life that could hardly have been lost by their descendants.
For this reason the dolichosaurs have usually been considered as
representing a distinct suborder. But they have many resemblances
otherwise to the monitors. They were semiaquatic in habit, and never
more than six feet in length. They are yet imperfectly known, and no
restoration of any form has hitherto been attempted. Their peculiar
interest lies in the elongation of the neck, quite like that of the
wholly unrelated nothosaurs and proganosaurs, which have been described
in the foregoing pages. Doubtless similar habits in each had like
results, but just what these habits were in the slender lizards we do
not yet know.
_Aigialosaurs._—Within recent years a number of other lizards have
been made known from the Lower Cretaceous rocks of Dalmatia which
present most remarkable intermediate characters between the monitors,
dolichosaurs, and the mosasaurs, the famous sea-lizards of Upper
Cretaceous age. Some of these lizards had twelve or thirteen vertebrae
in the neck, while others had but seven—an unusually short neck
characteristic of the mosasaurs. These latter kinds, belonging to two
or three genera, are included in a distinct group. They were long and
slender, the head long and pointed. The teeth, conical and sharp, were
attached in shallow pits, quite as in the mosasaurs. The lower jaws
had a hinge just back of the teeth, as in the mosasaurs, of which the
only trace in modern lizards is found among the monitors. Still more
remarkable, though perhaps not so easily appreciated, is the shape of
the quadrate bone, with a broad flaring rim for the ear cavity, quite
unlike that of land lizards, but quite like that of the mosasaurs. In
fact, the very peculiar skull is almost identical with that of the true
sea-lizards. The body and tail also resemble those of the mosasaurs
more than those of the monitors, but there is a firm attachment of the
pelvis to the backbone, and the legs are long and lizard-like, though
not as long as those of land lizards. The feet were webbed in life, and
the toes have no claws, conclusively demonstrating their water habits.
The vertebrae indeed have the same peculiar articulations, called
zygosphenes, as in most of the mosasaurs. The largest aigialosaurs were
about six feet in length, that is, of about the size of the smallest
known mosasaurs.
[Illustration: FIG. 68.—_Clidastes_, an American mosasaur. Life
restoration.]
We have then in the aigialosaurs nearly every known intermediate
character that we could wish for in a connecting link between the
mosasaurs and the monitors, lizards that were equally at home on land
or in the water, and there can be scarcely a doubt that they were
either the direct ancestors or closely akin to the direct ancestors of
the strictly marine mosasaurs; and scarcely a doubt that they were the
descendants of the actual forbears of the modern monitors, which, as we
have seen, have acquired partial aquatic habits in escaping from their
enemies. The dolichosaurs we can now understand were a side branch
from these semiaquatic aigialosaurs which, specializing in another
direction, quickly came to grief, perhaps in competition with their
more agile and skilful short-necked kin.
Taking all these facts into consideration it seems best to unite
the monitors, dolichosaurs, and aigialosaurs into one group of the
Lacertilia, the Platynota, intermediate in place between the true land
lizards and the truly aquatic mosasaurs.
MOSASAURS
At St. Pietersberg, a small mountain in the vicinity of Maestricht,
Holland, there are immense subterranean stone quarries, which have
been worked for more than a thousand years. The stone quarried from
them is a sandy limestone of Upper Cretaceous age containing many
well-preserved remains of extinct animals that have long been sought
by collectors of fossils. In 1776 Major Drouin—an officer of a near-by
garrison, one of much military importance in those days—secured from
one of these quarries some bones of an extinct reptile, which, though
of interest, afforded but little information concerning the structure
and affinities of the animal to which they had once belonged. In 1780
a very perfect skull, in excellent preservation, of the same kind of
an animal was obtained from the same quarry by Dr. Hofmann, an army
surgeon of the same garrison, whose interest in such things had been
incited by Major Drouin’s collections. This specimen, so renowned in
science, has had a remarkable and eventful human history, in part
related by St. Faujas de Fond, a French commissary of the “Army of the
North,” and one of the participants:
In one of the great galleries or subterranean quarries in
which the Cretaceous stone of St. Pieter’s Mount is worked,
about five hundred paces from the entrance, and ninety
feet below the surface, the quarrymen exposed part of the
skull of a large animal in a block of stone which they were
engaged in quarrying. On discovering it they suspended
their work and went to inform Dr. Hofmann, surgeon to the
forces at Maestricht, who for some years had been collecting
the fossils from the quarry, remunerating the workmen
liberally for the discovery and preservation of them. Dr.
Hofmann, arriving at the spot, saw with extreme pleasure
the indication of a magnificent specimen; he directed the
operations of the men, so that they worked out the block
without injury to the fossil, and he then, by degrees,
cleared away the yielding matrix and exposed the
extraordinary jaws and teeth, which have since been the
subject of so many drawings, descriptions, and discussions.
This fine specimen which Dr. Hofmann had transported with
so much satisfaction to his collection, soon became,
however, a source of much chagrin to him. Dr. Goddin, one
of the canons of Maestricht, who owned the surface of the
soil beneath which was the quarry whence the fossil was
obtained, when the fame of the fossil reached his ears,
pleaded certain feudal rights in support of his claim to
it. Hofmann resisted and the canon went to law. The whole
chapter supported their reverend brother, and the decree
ultimately went against the poor surgeon, who lost both the
specimen and his money, for he was made to pay the costs of
the action. The canon, leaving all remorse to the judges
who pronounced the iniquitous sentence, became the happy
and contented possessor of this unique example of its kind.
[Translation by Leidy.]
But the canon was ultimately despoiled of his ill-gotten treasure. At
the siege of Maestricht in 1795, the famous skull to which Hofmann
had devoted so much anxious thought and labor, fell into the hands of
the French and was carried off as one of the spoils of war. So widely
celebrated had the specimen become during the fifteen years which had
elapsed since its discovery, through the writings of several noted
scientific men, that the French general commanded his artillerists
to spare the house in which it was known to be. The canon, however,
shrewdly suspecting that such an unexpected and extraordinary mark of
favor was not for his own sake but rather for the sake of the famous
fossil, had it removed and carefully hidden in a house in the city.
After the capitulation of Maestricht the eagerly sought-for fossil was
not to be found, and the offer of a reward of six hundred bottles of
wine, so the story goes, was made for its recovery. So tempting was
the offer that, ere long, it was brought in triumph to the house of
St. Faujas de Fond, by a half-dozen grenadiers, whence it was later
transferred to Paris, where it now is.
We may well sympathize with Dr. Hofmann in the loss of his cherished
specimen, since, had it not been for his zeal, money, and labor,
it would never have escaped the usual fate of such things—complete
destruction. But we must remember that St. Faujas de Fond, the recorder
of this history, was a Frenchman, and somewhat interested in robbing
the reverend canon of it; possibly there is another side of the story
which has never been told.
After peace was declared, one has regretfully to add that the canon,
not Dr. Hofmann, was reimbursed for it, or so it is said. Cuvier rather
naïvely says that it was ceded to the Garden of Plants of Paris,
perhaps in the way that many other things are ceded to the conqueror
in time of war. The specimen is really a good one, even when compared
with many found in recent years, and there is little wonder that the
cupidity of St. Fond was incited by it. Casts of it are now or have
been in nearly every noted museum of the world, and pictures of it
illustrated nearly every textbook of geology published during the
first three-quarters of the past century. It had been the subject of
considerable controversy even before it came into the hands of Cuvier.
Peter Camper figured and described the skull as that of a whale or
“breathing fish”; while St. Fond himself later called it a crocodile.
Crocodile or alligator skeletons were rare in those days, and St. Fond
made a special trip to the British Museum to study one. But it was
really Adrian Camper, a son of Peter Camper, who deserves the credit,
so often wrongly ascribed to Cuvier, for the recognition of the true
nature of the fossil. He insisted that the animal was a lizard allied
to the living monitors, an opinion which it will be seen has finally
been proved to be correct within very recent years.
In 1808 this famous skull, and all other known remains of a similar
nature, came under the observation of Cuvier, the renowned French
naturalist and paleontologist, who confirmed the views of Adrian
Camper. He fully described and figured all the known parts of the
skeleton that had later come to light, calling the animal the great
lizard of the Meuse, the river near which Hofmann’s specimen was found.
Conybeare, a well-known paleontologist of England, some years later
formally christened it _Mosasaurus_, a transliteration of Cuvier’s
phrase, from the Latin _Mosa_, for Meuse, and _saurus_, a lizard. For
more than half a century Cuvier’s figure of the skull of the original
specimen appeared in works on geology over the name _Mosasaurus
hofmanni_, or _Mosasaurus camperi_. One could wish that the former name
for the species might prevail, in recognition of the zealous doctor who
was so shabbily treated in his possession of the specimen.
For some years the few specimens discovered by Drouin and Hofmann were
all that were known of the mosasaurs. A few others of related forms
were discovered in England, and some were reported from New Jersey by
early explorers, but there was little published about the mosasaurs
till 1843, when Dr. August Goldfuss, a noted German paleontologist,
described and beautifully figured an excellent specimen from the
United States. This specimen also had a rather eventful history.
It was discovered early in the fourth decade by Major O’Fallen, an
Indian agent, near the Great Bend of the Missouri River, whence it
was transported by him to St. Louis and placed in his garden as a
curiosity. It happened that Prince Maximilian of Wied, the famous
naturalist, in his travels through the United States, saw the specimen
and secured it, taking it to Germany on his return. He presented it
to the Museum of Haarlem where Goldfuss saw and described it. Rather
oddly, this specimen was of a species closely allied to the original
one of Maestricht, a species which has since only rarely been found.
It was called _Mosasaurus maximiliani_ by Goldfuss, though some time
previously, it has since been found, some fragments of the same
species were described by Harlan, an American author, under the name
_Ichthyosaurus missouriensis_. Goldfuss’ paper was strangely overlooked
by subsequent writers, and it was not till the discovery of numerous
remains of mosasaurs by Leidy, Cope, and Marsh in the chalk of western
Kansas, nearly thirty years later, that much was added to the world’s
knowledge of these strange reptiles.
[Illustration: FIG. 69.—Skeleton of _Plalecarpus_ as mounted in the
Paleontological Exhibit. Walker Geological Museum, University of
Chicago.]
Perhaps nowhere in the world are the fossil remains of marine animals
more abundantly and better preserved than in these famous chalk
deposits of Kansas. The exposures are of great extent—hundreds of
square miles—and the fossil treasures they contain seem inexhaustible.
Long-continued explorations by collectors have brought to light
thousands of specimens of these swimming lizards, some of them of
extraordinary completeness and perfect preservation, so complete and
so perfect that there is scarcely anything concerning the mosasaurs
which one might hope to learn from their fossil remains that has not
been yielded up by these many specimens. The complete structure and
relations of all parts of the skeleton, impressions of the bodies made
in the soft sediments before decomposition had occurred, the character
of their food, the nature of the skin covering, and even some of the
color markings of the living animals have all been determined with
certainty. Not only from Kansas, but also from many other parts of the
world, have remains of these animals been discovered, until now it may
truthfully be said that no other group of extinct reptiles is better
represented by known fossil remains than the mosasaurs. From England,
Belgium, Russia, and France in Europe; from New Jersey, Georgia,
Alabama, Mississippi, Texas, New Mexico, Colorado, Kansas, Nebraska,
the Dakotas, Wyoming, and other places in the United States; from New
Zealand and South America they have been obtained in greater or less
abundance and perfection.
[Illustration: FIG. 70.—Tooth of _Tylosaurus_.]
Their geological history is relatively brief, notwithstanding their
wide distribution over the earth in such great numbers and diversity.
The earliest are known from near the beginning of the Upper Cretaceous
of New Zealand, whence it is believed by some that they migrated to
other parts of the world, appearing in North America some time later.
They reached their culmination in size, numbers, and variety very soon,
and then disappeared forever before the close of Cretaceous time. The
largest complete specimen of a mosasaur known measures a little more
than thirty feet in length, but incomplete skeletons of others indicate
a maximum length of about forty feet. The skulls of the largest species
are about five feet long. The smallest known adult skeletons are
scarcely eight feet in length. There are now known at the present time
seven or eight genera of three distinct types, all belonging to one
family, the Mosasauridae, including about twenty-five known species.
While a few of the genera are widely distributed over the earth, the
species are all of restricted range, indicating, perhaps, non-migratory
habits.
The adaptation of the mosasaurs to an aquatic life was very complete,
though perhaps not so complete as was that of the ichthyosaurs. The
skull is flattened, narrow, and more or less elongate, but large in
proportion to the remainder of the skeleton—nearly one-sixth of the
entire length; that relative size doubtless is indicative of very
predaceous and pugnacious habits. The teeth in the typical forms are
numerous, strong, and sharp, conical in shape, and recurved. Not
only are there numerous teeth in both the upper and lower jaws, but
there are also two rows of strong teeth implanted in the back part of
the palate, upon bones called pterygoids, the use of which will be
understood later. The teeth were inserted on large, tumid, bony bases,
rather loosely attached in shallow pits or alveoli, unlike the teeth of
all modern lizards. Such a mode of attachment of the teeth doubtless
had some relation to the habits of the animals concerning which we
are not quite clear. They were easily dislodged, and, in consequence,
of very unequal size, some full grown, some small, and others just
appearing above the surface of the gums in the living animals. The
frequent loss of teeth and their constant and easy replacement by new
ones is a peculiarity of predaceous reptiles, thereby insuring their
best functional use.
The external nostrils, of large size, were situated at a considerable
distance back of the end of the snout, but not nearly so far back or so
near the eyes as were the nostrils of the ichthyosaurs, plesiosaurs,
and phytosaurs. Their size and position suggest a use like that of the
modern aquatic monitors, as mentioned on a preceding page. The eyes
were of moderate size, those of the less purely aquatic forms being
directed more laterally than those of species of more distinctly diving
habits. They were protected by a stout ring of bony plates, as were the
eyes of all truly aquatic reptiles of the past. The ears, also, in most
if not all mosasaurs, had a thick cartilaginous ear-drum in place of a
simple membrane, evidently, as Dollo has shown, for better protection
under undue pressure of the water in deep diving.
[Illustration: FIG. 71.—_Clidastes_, inner side of right mandible:
_ang_, angular; _art_, articular; _cor_, coronoid; _pa_, prearticular;
_sur_, surangular.]
As in all other lizards, the bones with which the lower jaws
articulate, the quadrates, were loosely attached at the upper end,
permitting great freedom of movement in all directions, more even than
the land lizards have. The lower jaws were long and powerful, armed
with a single row of teeth on each side, from sixteen to eighteen
in number. Just back of the teeth, a little beyond the middle, each
mandible has a remarkable joint, quite unknown in land lizards, though
a trace of it is found in the monitors, permitting much movement
between the front and back parts, both laterally and vertically, though
chiefly in the former direction. Furthermore, as in land snakes but not
as in land lizards, the front ends of the two sides of the jaws were
somewhat loosely attached to each other by ligaments. This looseness of
the two sides of the jaws, not only in front but also behind, together
with the joint in each, was of the greatest use in swallowing prey, as
will be explained farther on.
[Illustration: FIG. 72.—Skulls of mosasaurs. Upper figure, _Clidastes_,
from the side; middle figure, _Platecarpus_, from below; lower figure,
_Tylosaurus_, from above: _an_, angular; _bs_, basisphenoid; _c_,
coronoid; _ep_, epipterygoid; _fr_, frontal; _j_, jugal; _l_, lacrimal;
_m_, maxilla; _na_, nasal; _oc_, occipital condyle; _pa_, parietal,
palatine; _pm_, premaxilla; _pf_, prefrontal; _pt_, pterygoid; _po_,
postorbital; _q_, quadrate; _sp_, splenial; _sq_, squamosal; _tr_,
transverse; _v_, vomer.]
As in most other aquatic reptiles, the neck was short and strong, the
vertebrae being less in number than in most other lizards. The trunk
was long and slender, more especially so in the surface-swimming kinds,
with from twenty-two to thirty-four vertebrae. The tail was long, no
longer than the tail of some land lizards, but more powerful, and
broader and flatter. It was expanded or dilated more or less toward the
free end, that is, with the beginning of a terminal caudal fin, such
as the more specialized ichthyosaurs and crocodiles possessed. The
vertebrae were procoelous, that is, concave in front and convex behind,
like those of most modern lizards and all modern snakes and crocodiles,
but quite unlike the biconcave vertebrae of all other aquatic reptiles.
This kind of articulation of the backbones gave greater firmness and
strength to the spinal column, but decreased the flexibility, and its
possession by these animals was doubtless due to their descent from
land lizards which had already acquired it. The loss of flexibility,
however, was partly compensated by the loss of the additional
articulating surfaces of the tail.
[Illustration: FIG. 73.—_Platecarpus_; occipital view of skull: _bo_,
basioccipital; _eo_, exoccipital; _pf_, postfrontal; _st_, stapes;
_pt_, pterygoid; _q_, quadrate.]
As in all other aquatic reptiles, it is in the limbs that the most
striking characteristics of these water lizards or “sea-serpents” are
found. The legs were so completely adapted to an aquatic mode of living
that the animals must have been practically helpless upon land, able
perhaps to move about in a serpentine way when accidentally stranded
upon the beaches, but probably never seeking the land voluntarily. The
front limbs, like those of all other swimming animals having a powerful
propelling tail, were larger than the hind ones, though not very much
so. The bones of the first two segments, that is, the arm, forearm,
and thigh and leg bones, were all short and broad, resembling those of
the ichthyosaurs more than those of any other reptiles, save perhaps
the thalattosaurs, discussed below. The articular surfaces of all the
limb bones, as in other aquatic animals, were restricted in extent,
indicating limited motion between the joints, though doubtless having
great flexibility. In the most specialized types, such as _Tylosaurus_,
the wrist and ankle bones were almost wholly cartilaginous, just as
they are in the water salamanders, and in whales and porpoises. This
tendency of the ends of long bones, the wrists and ankles as well as
other bones of the skeleton, to become more cartilaginous, or less
well ossified, in animals purely aquatic in habit is a marked one. So
much is this the case that paleontologists always suspect water habits
in reptiles showing it, even though but few parts of the skeleton are
known.
[Illustration: FIG. 74.—_Clidastes_; left front paddle: _c_, coracoid;
_h_, humerus; _r_, radius; _sc_, scapula; _u_, ulna.]
[Illustration: FIG. 75.—_Tylosaurus_; left front paddle: _c_, coracoid;
_sc_, scapula; _h_, humerus; _r_, radius; _u_, ulna.]
Increase in the number of bones of the digits is a more or less
conspicuous characteristic of all mosasaurs. In those forms in which
the wrists and ankle bones had become cartilaginous in great part, as
many as eleven phalanges have been observed in the longest toes, though
in other forms, those with more completely ossified wrists and ankles,
only two or three additional bones have been developed in the longest
fingers and toes by aquatic habits. The pliability and flexibility of
the fingers and toes were certainly very great, but they could not
possibly have been flexed or bent so as to grasp or seize anything;
and of course all vestiges of claws had disappeared. Many specimens
have been found with all the bones of the limbs, that is, the “paddle
bones,” in the positions they occupied when the animals died. Figures
of three such specimens, made from photographs or careful drawings by
the writer, are shown herewith (Figs. 74-76). In several such specimens
very clear impressions of the smooth membranes between the fingers have
been observed, and in one specimen preserved in the collections of the
University of Kansas the outline of the fleshy parts connecting the
paddle with the body has been preserved.
[Illustration: FIG. 76.—_Platecarpus_; right front paddle: _h_,
humerus; _r_, radius; _u_, ulna.]
It will be seen by comparison of the figures of the mosasaur paddles
with those of the ichthyosaurs and plesiosaurs that there was a wide
difference in their structure, though all have the characteristic
shortening of the limb bones and increase in the numbers of the
finger and toebones, that is hyperphalangy. It is probable that these
differences mean a more powerful and varied use of the limbs in the
mosasaurs. It is certain that the mosasaurs were much more predaceous
and pugnacious in their habits than were any other truly aquatic
backboned air-breathing animals of the past or present. They were the
“land sharks” of the ancient seas, and probably the only ones among
water reptiles that would be dangerous and offensive to man, were they
all living today.
For a long time it was thought that the mosasaurs had no breast
bone, and that, in consequence, the front part of the thorax was
expansible. Under this assumption the mosasaurs would have been much
more snake-like in habit than they really were. The loose construction
of the jaws doubtless permitted the swallowing of prey of considerable
size, and the inference was that they habitually preyed upon animals of
large size. A snake will often swallow a frog of larger diameter than
its own body, the flexible jaws and loosely connected ribs permitting
it to pass to the abdominal cavity. But the unyielding ring formed by
the anterior ribs connected with the breast bone in the mosasaurs, as
in other lizards, conclusively proves that large animals could not
have been swallowed whole by the mosasaurs. In several instances the
fossilized stomach contents, composed chiefly or wholly of fishes, have
been found between the ribs of mosasaurs, and in none were the fishes
more than two or three feet in length, though the reptiles were from
sixteen to twenty feet long. Possibly the largest mosasaurs, those
thirty or thirty-five feet in length, might have captured and swallowed
fishes six or seven feet long, but in all probability their usual prey
was of smaller relative size.
[Illustration: FIG. 77.—_Platecarpus_; pelvis, from below: _p_, pubis;
_il_, ilium; _is_, ischium.]
The very loose construction of the pelvic bones, those to which
the hind legs are articulated, is an evidence of more complete
adaptation to water life than was or is the case with any other water
air-breathers except the ichthyosaurs and cetaceans. The sacrum
had entirely lost its function as a support to the pelvis and had
disappeared, that is, the vertebrae composing it had become quite
like the adjacent ones, by the loss of the ribs connecting them with
the ilium. The small pelvis was suspended loosely in the walls of the
abdomen, or at the most was feebly connected with a single vertebra
by ligaments. It was entirely useless as a support for the legs.
The mosasaurs could not possibly have raised their bodies from the
ground while on land. It is well known that the land lizards and the
crocodiles raise their bodies free from the ground while running or
walking; none drags its body over the surface.
[Illustration: FIG. 78.—Photograph of carbonized remains of scales of
_Tylosaurus_.]
In several instances complete or nearly complete skeletons of mosasaurs
have been discovered with the different bones nearly all in the
positions and relations they had after the decomposition of the flesh,
together with the carbonized remains of the skin and impressions of
the investing scales and membranes. The nature of the body covering is
therefore known with certainty from nearly all parts of the body. The
body everywhere, save on the membrane between the fingers and toes, and
perhaps on the top of the skull, was covered with small overlapping
scales, very much like those of the monitors. These scales, however,
were small and smooth in comparison with the size of the animals, those
of a mosasaur twenty feet in length being almost precisely the size of
those of a monitor six feet long. The top of the skull seems to have
been covered with horny plates, as in most lizards. In one instance
parallel dark bars, obliquely placed, and of narrow width, formed by
carbonized pigment, were observed by the writer. As has been stated,
in some instances fish bones and fish scales have been observed among
the fossilized stomach contents, and it is quite certain that the
food of these creatures must have been composed chiefly of fishes,
though of course it is not improbable that other small vertebrates,
birds, pterodactyls, the young of plesiosaurs, and possibly small
mammals, may occasionally have formed a part of their diet. That the
mosasaurs were very pugnacious in life is conclusively proved by the
many mutilations of their bones that have been observed, mutilations
received during life and partly or wholly healed at the time of death.
Bones of all vertebrates are repaired after injury by the growth of
more or less spongy osseous material about the injured part, forming a
sort of natural splint. This material is more or less entirely removed
by absorption when it is no longer required for the support of the
broken ends. Many such injured bones of the mosasaurs have been found;
sometimes the bones of the hands and feet have grown together, and not
infrequently the vertebrae have been found united by these osseous
splints; occasionally even the skull itself, especially the jaws,
attest extensive ante-mortem injuries. In a single instance the writer
has observed the loss of a part of the tail, where it probably had been
bitten off. It may be mentioned, however, that the bones of the tail
had no such “breaking points” in the mosasaurs as have those of many
land lizards, whereby a part or all may be lost as a result of even a
trivial injury, and then regrown. Such a condition in an organ relied
upon entirely for propulsion would have been immediately fatal to the
existence of the mosasaurs. The large jaws and teeth are in themselves
sufficient evidence of the fiercely carnivorous propensities of the
mosasaurs. The constant renewal of the sharply pointed teeth, thereby
preventing deterioration by use or accident, preserved, even in the
oldest animals, the effectiveness of the youthful structure.
We may now understand how the mosasaurs seized and swallowed their
prey. Living constantly in the water, away from all firm objects, with
small, short limbs quite incapable of holding struggling prey, and
the body not sufficiently serpentine to hold it in its folds after
the manner of snakes, the mosasaurs would have found it difficult or
impossible to swallow fishes of even moderate size, were their jaws of
the same construction as are those of the land lizards. If they preyed
upon small animals only, or if they tore their prey to pieces after the
manner of the alligators, there would have been no especial difficulty
in deglutition. But it is certain that the animals which the mosasaurs
devoured were not always small, and they must have been swallowed
whole, since their teeth were not adapted, like those of the alligators
and true crocodiles, for the rending of bodies. One who has watched a
snake swallow a frog or another snake will appreciate the difficulties
against which the mosasaurs contended in swallowing fishes a fifth or a
sixth of the length of their own bodies. The ordinary snake, no matter
where or how it seizes its prey, invariably swallows it head first. Its
mandibles are even more loosely united in front than were those of the
mosasaurs, and while there is no joint in the snake’s mandibles such
as there is in the mosasaurs’, the loose union of the various bones of
the mandibles serves the same purpose. The frog or lizard, while firmly
held by the slender teeth, is slowly moved sideways by the alternate
lateral action of the jaws till the head is reached. Many non-poisonous
snakes, if they find it impracticable or impossible to reverse the
position of their prey in this way, wrap the folds of their body about
it, holding it firmly while they release their mouth-hold and seize
it by the head. An amusing instance of these habits came under the
observation of the writer not long ago, in Texas. A large “blue racer”
(_Bascanion_), six feet four inches in length, caught an unusually
large bullfrog by one hind leg, but in almost less time than it takes
to relate, the head of the frog had entered the snake’s gullet and the
mouth was closed over it, notwithstanding the vigorous muscular and
vocal protests on the part of the frog. Wishing to secure the skull
of the snake for his collection, the writer seized an ax standing
conveniently by and cut the snake cleanly in two. The peristaltic
action of the deglutitional muscles carried the frog slowly on about
two feet farther to the ax-made orifice, from which it emerged, and,
after a few croakings against such unkind usage, calmly hopped off into
the near-by pool of water! Many poisonous snakes release their prey
after killing it; other snakes may force their prey down the throat by
pressing it against the ground.
Even small fishes could not possibly have been swallowed by the
mosasaurs in any other way than head first, since the backwardly
projecting, and often long, spines would have rendered any other
procedure impossible. Even after the head had entered the gullet,
deglutition could have been effectively completed only by the aid of
some mechanism whereby the fish could have been pulled or pushed back
into the constricting fauces. The strong teeth of the upper jaws and
palate held firmly the struggling prey, while the loosely united jaws,
bending laterally at the joint back of the middle, either alternately,
or more probably in unison, steadily forced it far enough back to be
seized by the muscles of the fauces.
The shape of the mosasaurs, though slender, does not suggest
extraordinary speed in the water; doubtless most of the fishes that
lived in the seas with them could swim faster than they. Their prey
was captured, for the most part at least, by sudden and quick lateral
movements, for which their powerful and flexible paddles admirably
adapted them.
It is a rather remarkable fact that, among the thousands of specimens
of mosasaurs which have been collected during the past forty years in
both Europe and North America, there never has been found one of a
very young animal. Of almost all other animals occurring abundantly
as fossils some specimens are sure to be discovered of young and even
embryonic individuals. It is quite certain that all such voracious
monsters as were the mosasaurs did not die of old age. Some specimens,
it is true, have been found that were evidently not full grown animals,
but the observed differences in the size of the fossil bones are not
great. All are of adult or nearly adult animals. If the mosasaurs were
oviparous, as were the ichthyosaurs, and probably the plesiosaurs,
and as are some living land lizards, the apparently entire absence of
embryonic bones associated with often nearly complete skeletons of the
mosasaurs is inexplicable; certainly _some_ mosasaurs must have died
a short time before the birth of their young. But embryos have never
been discovered, though numerous skeletons inclosing fossilized stomach
contents have been found. From this fact it would seem very probable
that the mosasaurs were oviparous, as are most other lizards. But this,
after all, may be a hasty inference.
No known reptiles lay their eggs in the water. Perhaps there is some
reason why the eggs of reptiles and birds, so different from those of
fishes and amphibians, cannot hatch in water; and there is no good
reason for supposing that the mosasaurs were exceptions to this rule.
Unless carefully hidden or protected by the parent, the eggs or very
young of the mosasaurs would have been subject to many and grave
dangers. Fish eggs are usually small and produced in great numbers,
thousands often being extruded from a single female. Among so many
there is a greater probability that at least two will hatch and survive
to maturity, reproducing their kind. It is unreasonable to suppose
that the lizards of the past were more prolific of eggs than are their
relatives now living; nor is it possible that their eggs could have
been as small as are those of most fishes. Modern lizards seldom lay
more than twenty-five or thirty eggs at a time; even the turtles, with
their greater vicissitudes, seldom produce more than one hundred. The
eggs of the mosasaurs were certainly large and few in number, and the
young animals must have begun breathing air immediately after escaping
from the shells. If the mosasaurs were oviparous they must have laid
their eggs upon the shores and beaches, as do the sea-turtles and the
Crocodilia. Nor is it at all probable that the female mosasaurs gave
even that protection to their eggs or young that the crocodiles and
turtles give. The young mosasaurs, perhaps reaching a foot in length,
must have been left entirely to their own devices and their own fate at
the very earliest stages of their independent careers.
The waters in which the mosasaurs abounded swarmed with many kinds of
predaceous fishes, to say nothing of the hordes of their own kinds,
all carnivorous in the highest degree, to all of which the tender
saurians must have been choice food. Possibly the shallow waters of the
bays and estuaries may have afforded protection to the newly hatched
reptiles. It would seem probable that the female mosasaurs went up
the rivers for a shorter or longer distance to lay their eggs or give
birth to their young, and that the young reptiles remained in such
relatively protected places until of a sufficient size to cope with
the fierce enemies of the open seas. We know practically nothing of
the inhabitants of the lakes and rivers during all the time in which
the mosasaurs existed; and this perhaps is the real reason why we have
never yet found a specimen of a young mosasaur.
[Illustration: FIG. 79.—Head of _Tylosaurus_.]
We have seen that many skeletons of ichthyosaurs are found entire, and
but little disturbed in position, suggesting, if not proving, that the
animals as a rule lived and died far out in the deep seas, away from
the disturbing effects of currents of water on their decaying bodies.
Among the thousands collected, the great majority of the specimens
of mosasaurs consist of a few bones or a part of the skeleton only.
Moreover, nearly all specimens show the disturbing effects of currents
of water; and the bones are usually associated with those of turtles,
birds, and flying reptiles, which probably did not often venture far
from the shores; all of which goes to prove that the mosasaurs in
general lived in the comparatively shallow waters of the seas, and
not far from the shores. That some were excellent divers, descending
probably many fathoms deep in the water, is certain, because of the
extraordinary protective structures of the eyes and ears.
But the various kinds of mosasaurs differed not a little in their
habits. Some, like _Mosasaurus_ and _Clidastes_, were doubtless chiefly
surface swimmers, as is evidenced by their better ossified bones,
firmer articulation, and the presence of the additional zygosphenal
articulations of the vertebrae, wanting in other forms, as also by the
structure of their paddles. They had a relatively long body and short
tail, the tail having a more pronounced distal expansion than in the
case of other forms, and the eyes looking laterally, not at all upward.
The feet, as shown in Fig. 74, were broad and short, with most of the
wrist and ankle bones well ossified, and with but few extra bones in
the digits. _Tylosaurus_ (Fig. 79), on the other hand, had a more
slender skull, the nostrils were situated farther back from the tip of
the snout, the tail was longer and more powerful, and the feet were
very highly specialized (Fig. 75). The wrist and the ankle were almost
wholly cartilaginous, the fifth finger and fifth toe were much longer,
and the number of phalanges was greatly increased. Moreover, the bones
of the skeleton are more spongy, the joints are more cartilaginous,
and the ears were better protected by a heavy coat of cartilage. In
most of these respects the genus _Platecarpus_ was intermediate between
_Clidastes_ and _Tylosaurus_ (Fig. 76).
Like nearly all other lizards, the mosasaurs had a pineal opening
in the skull, but it is not at all probable that they possessed a
functional pineal eye.
Many and varied have been the opinions of scientific men regarding
the relationships of these animals, as has been intimated. They were
thought to be a kind of whale or breathing fish by Peter Camper;
crocodiles, by St. Fond; and aquatic lizards, by Adrian Camper and
Cuvier. The late Professor Cope thought they were more nearly related
to the snakes than to the lizards, and that they might even have been
the ancestral stock from which the snakes have descended. Because
of this belief he gave to them the name Pythonomorpha, meaning
python-like, and this name, really the first ever applied to them, is
yet often used instead of Mosasauria. A more complete knowledge of
the mosasaurs, however, and especially the recent discoveries of the
semiaquatic connecting links, called the aigialosaurs and described
on a preceding page, have set at rest all doubt as to their real
affinities. They are real lizards, differing less from the living
monitor land lizards than do the monitors from some other land lizards,
especially the amphisbaenas and chameleons. And to Adrian Camper is
due the credit for the recognition of their real relationship, though
it required more than a century to prove that he was right.
[Illustration: FIG. 80.—_Globidens alabamensis._ Part of mandible, with
teeth. (From Gilmore.)]
Very recently, and since the foregoing was written, a remarkable new
type of mosasaurs has been discovered in Alabama and Europe. Only
fragmentary jaws, a few vertebrae, and some skull bones are known,
so that it is impossible yet to decide how closely the new form is
related to the true mosasaurs, but so far as the evidence goes the
only distinguishable character is the teeth. These, instead of being
elongated and pointed, are nearly spherical, as shown in Fig. 80. Such
teeth could have been used only for crushing shell-fish, and not at all
for the seizure and retention of slippery fishes. The genus, which was
called _Globidens_ by its discoverer, Mr. Gilmore, includes two known
species, from Alabama and Europe, the latter recently described by
Dollo. It has been suggested that this peculiar kind of dentition was a
more primitive or intermediate one, a kind that the first mosasaurs had
before they became fully adapted to the water; but this is doubtful,
since _Globidens_ comes from late Cretaceous, and must be one of the
later types. If _Globidens_ is a true mosasaur, and it seems to be one,
its life-habits must have been remarkably different from those that
have long been known. Possibly when the limbs and more of the skull are
found, _Globidens_ will prove to be of a distinctive type.
SNAKES
The chief differences between snakes and lizards have already been
given and need not be repeated, save very briefly. Snakes are always
functionally legless, though some have vestiges of the hind pair; the
brain-case is wholly bony; the upper temporal bar is wanting; the
lower jaws are united in front by ligaments only, like those of the
mosasaurs; the vertebrae are greatly increased in number, and always
have the additional zygosphenal articulations like those of _Clidastes_
and _Mosasaurus_ and some lizards; there is but one lung, and the eyes
are always without free eyelids. But these characters are really not
very important, since every one of them is found in the lizards or
mosasaurs, except the complete ossification of the brain-case, and even
this is partly ossified in the mosasaurs. It is rather the presence of
all these characters which distinguishes a snake from a lizard.
The number of living snakes is nearly as great as that of the living
lizards, and their distribution over the earth is very similar. Snakes
are for the most part strictly terrestrial in habit. Some live more
or less among trees, and some live in the water, though with but few
exceptions all are fully capable of rapid progression upon land. They
are almost invariably carnivorous in habit, swallowing their prey
whole, and usually alive, as has been described. Some poison their prey
or crush it to death before swallowing it. Some feed upon eggs which
are swallowed whole and then crushed in their stomachs by projecting
bones from the under side of the vertebrae developed for that purpose.
In size snakes vary from a few inches in length to twenty-five or more
feet, no known extinct forms being larger than the living anacondas
and boas. In geological history the earliest remains known date from
the latter part of the Cretaceous, and it is quite probable that they
have a briefer history than that of the lizards of which they are the
descendants. Venomous serpents are known only from comparatively recent
geological times, and it is probable that venomosity is the latest and
final specialization of importance in the reptilian class.
[Illustration: FIG. 81.—_Hydrus bicolor_; sea-snake. (From Brehm)]
Of strictly aquatic snakes there is no known geological history, and it
is improbable that there is any such history. There are a few snakes
now living—very venomous ones, allied to the deadly cobras—which
have become so completely adapted to life in the water that they are
unable to exist or even move about on land. These are the well-known
sea-snakes of the Indian Ocean and adjacent waters. Perhaps the most
highly specialized and typical of these is the black-banded sea-snake,
_Distina cyanocincta_, which reaches a length of four or five feet,
and is a rapid and excellent swimmer. From the figure (Fig. 81) it is
seen that the body is very much flattened from side to side, and lacks
or has but a few vestiges of the transverse scales on the under side
so characteristic of all other snakes, and which enable them to move
about on land. So helpless are these snakes on land that it is said
sailors will handle them carelessly, because of their inability to bite
while out of water, though the bite is very venomous. They never come
on land for any purpose whatever, and their young, unlike those of
most other snakes, are born alive. There are a number of species of
these sea-snakes, though comparatively little is known of their habits.
They are of especial interest as another example of the ways in which
air-breathing land vertebrates have become adapted to water life. The
adaptation, however, was simple, for nearly all snakes swim freely in
water by undulatory movements; it would require not much change to
convert an ordinary water snake into one like these sea-snakes.