On the dermal armour of _Jacare_ and _Caiman_, with notes on the
Specific and Generic Characters of recent _Crocodilia_. By T. H. HUXLEY,
Esq., F.R.S., F.L.S., Prof. of Nat. History, Gov. School of Mines.
[Read Feb. 17th, 1859.]
In the course of a recent investigation into the nature of the singular
extinct reptile, _Stagonolepis_, I was led to inquire somewhat
minutely into the character of the exoskeleton, or dermal armour, of
the existing _Crocodilia_. To my surprise, I found that very little
detailed information on the subject was to be obtained from the standard
repertories of Comparative Anatomy, or even from the special monographs
on Crocodilian structure and classification; but I was still more
astonished to discover, among whole genera of recent _Crocodilia_,
an exoskeleton possessed of characters such as have been universally
supposed to be peculiar to long extinct forms of the order, and whose
existence in any recent species has hitherto, so far as I can ascertain,
been completely overlooked.
The attempt to discover the limits within which this remarkable
exoskeleton is to be found, led me to look, more critically than I had
previously done, into the arrangement and specific characterization
of the recent _Crocodilia_. I have thereby arrived at results which,
imperfect as they are, may be of service by leading others to inquire
into the exact characters of species not at present within my reach;
and I therefore propose to preface my account of the peculiarities of
the exoskeleton in two of the genera of recent Crocodiles with some
remarks on the classification of the group, and with a few notes upon
the characters of the species and the limits of the genera.
Everyone is acquainted with the great improvement effected in this
branch of Herpetology by Cuvier, who divided the Crocodiles, which he
regarded as constituting only a single genus, into the three subgenera
_Alligatores_, _Crocodili_, and _Longirostres_. Subsequent writers
have admitted these highly natural subdivisions; but there has been a
constant tendency to raise their rank. The genus _Crocodilus_ has become
the order _Crocodilia_; the subgenera _Alligatores_, &c., have been
elevated into families; Dr. Gray has shown that the _Alligatores_ must
be divided into three genera, and that there are at least two genera of
_Crocodili_; and, while one of Cuvier's species of _Longirostres_ has
been suppressed, the group is very generally retained with a changed
name (_Gavialis_), a very important addition having been made to it in
the _Crocodilus Schlegelii_ of Müller and Schlegel.
Unless the considerable materials contained in the British Museum, the
Hunterian collection, the collection of Dr. Grant, and the Christchurch
Museum at Oxford had been freely placed at my disposal, I should have
been wholly unable to acquire the information contained in the following
pages. It is only right, therefore, that I should take this opportunity
of offering my thanks to my friends Dr. Gray, Prof. Quekett, Dr. Grant,
and Dr. Rolleston for the many facilities they have liberally afforded
me.
The recent species of the order _Crocodilia_ are divisible into three
families, which correspond with the original subgenera of Cuvier, and
may be termed the _Alligatoridæ_, the _Crocodilidæ_, and the
_Gavialidæ_.
I. In the ALLIGATORIDÆ the teeth are strong and unequal, and the
posterior ones differ greatly in shape from the anterior. The anterior
pair of mandibular teeth, and the fourth pair (or the so-called canines)
are received into pits in the margins of the premaxilla and maxilla;
while the mandibular teeth behind these pass inside, and not between,
the maxillary teeth. The mandibular symphysis does not extend back
beyond the level of the fifth tooth, and often not nearly so far. The
line of the premaxillo-maxillary suture on the palate is straight,
or convex forwards. The wide posterior nares look downwards, and are
situated forwards on the palate.
This family embraces three genera, readily distinguishable by
osteological characters--_Alligator_, _Caiman_, and _Jacare_.
Genus 1. ALLIGATOR.
Dental formula, 20-20/20-20. 9th maxillary tooth the largest of
its series. The snout is very broad, flattened, and rounded at the
end. There is an indistinct longitudinal interorbital ridge; and there
are two short ridges along the line of junction of the prefrontal
and lachrymal bones. The aperture of the external nares is divided
into two parts, by the prolongation forwards of the nasal bones. The
supra-temporal fossæ are well-marked and open, though not large. The
vomers do not appear in the palate. The feet are well webbed. The dorsal
bony scutes are not articulated together; and there are no ventral
scutes.
This genus contains only one species, the well-known _Alligator
Mississipiensis_, or _lucius_, which is exclusively North American.
Cuvier (Oss. Foss. ed. 4. vol. ix. p. 211) gives the appearance of the
vomer in the palate as a general character of the _Alligatores_; but
this bone is not visible in the palate of any of those _Alligatores_
which Cuvier would have referred to his _A. lucius_ or _A. palpebrosus_,
and which form the genera _Alligator_ and _Caiman_ as here defined. The
vomers are in fact as slender and delicate as in the Crocodile, and
extend only between the level of the tenth maxillary tooth anteriorly
and the descending processes of the prefrontal posteriorly.
What may be called the median nares, or the arch formed by the
postero-lateral part of the vomer and the anterior and superior lamina
of the palatine bone on each side (which would constitute the posterior
boundary of the posterior nares, if the palatine and pterygoid bones
gave off no inferior or palatine processes), are situated nearly on a
level with the twelfth tooth, or with the palato-maxillary suture.
Genus 2. CAIMAN.
Dental formula 20-20/22-22 (Natterer). The face is without median
or transverse ridges, but it is sharply angulated along a line which
extends from the orbit forwards along the sides of the snout. The
anterior nasal aperture is undivided in the dry skull. The vomers do
not appear in the palate. The supra-temporal fossæ are obliterated,
the circumjacent bones uniting over them. The webs of the feet are
rudimentary. The dorsal scutes are articulated together by lateral
sutures and anterior and posterior facets; and there is a ventral
shield, consisting of similarly articulated scutes.
Natterer[1] has described three species of _Caiman_--_C. palpebrosus_,
_C. trigonatus_, and _C. gibbiceps_. The Caimans abound chiefly in
tropical South America; but they are found as far north as Mexico, a
specimen of _C. palpebrosus_ in Dr. Grant's collection coming from that
country.
Genus 3. JACARE.
The snout is broad, and rounded at the end[2]. Each prefrontal bone
is traversed close to its anterior extremity by the ends of a strong
transverse ridge, which then curve round and pass forwards on the
lachrymal and maxillary bones, to subside opposite the ninth tooth.
The anterior nasal aperture is not divided by bone. The vomers,
separated by a longitudinal suture, appear in the palate between the
premaxillaries and the palatine plates of the maxillaries. The temporal
fossæ, though not large, are open. The webs of the feet are small. The
dorsal scutes are articulated together, as in the preceding genus; and
there are similarly-articulated ventral scutes. There are 18-20 teeth
on each side, above and below; and the fourth tooth in the upper jaw is
the largest. The mandibular symphysis extends back nearly to the fifth
tooth.
In a skull of _Jacare_ (_fissipes?_), 19 inches long, in the British
Museum, I find that part of the vomer which is visible in the palate to
be a rhomboidal plate, somewhat truncated anteriorly, and rather more
than 1-1/2 inch long and 1 inch wide. Its anterior end comes within
3/8ths of an inch of the posterior margin of the anterior palatal
foramen. Its posterior margin reaches to the level of the eighth tooth.
The visible portion of each vomer is only its anterior end, which forms
a thick and solid wedge-shaped plate, broader in front than behind, and
articulating by a rough anterior and outer face with the premaxilla, by
an obliquely ridged posterior and outer face with the maxilla, and by
its internal face with its fellow. Its upper, rounded surface projects
but little into the nasal passage. 2-1/4 inches behind its anterior
end, the posterior and upper extremity of the vomer passes into a thin
and narrow plate of bone, whose plane is at first inclined at an angle
of 45° to that of the anterior part of the bone, but gradually becomes
vertical; as it does so it deepens, until, 3 inches behind the anterior
extremity, the vomer is a thin vertical plate of bone, 5/8ths of
an inch deep, which articulates below with the palatine plate of the
maxilla, and, about 1 inch behind this, with the palatine plate of the
palatine bone. The upper edge of this plate nowhere extends to one-third
of the height of the nasal chamber. It gives off a horizontal process
outwards, which, gradually increasing in width, inclines downwards until
it comes into contact, first, with the inner surface of the maxilla,
and, 3/4ths of an inch behind this, with the nasal plate of the
palatine bone. In front of its junction with the maxilla, the horizontal
plate of the vomer presents a long free edge, concave externally; and
this bounds the median nares internally and posteriorly. Throughout its
junction with the maxilla, the horizontal plate is parallel-sided; but
after it joins the palatine bone, it gradually narrows posteriorly,
in consequence of the gradual increase in width of the palatine, and
ends almost in a point, 6-1/4 inches behind its anterior end. The
posterior edge of the vertical plate is extremely thin, and 7/8ths
of an inch deep. It articulates with the anterior end of the vertical
plate of the pterygoid, while the straight inferior edge articulates
throughout with the palatine plate of the palatine bone. The vomers
terminate midway between the median nares and the descending process of
the prefrontal. The median nares are bounded entirely by the vomer and
the maxilla. They correspond with the nasal face of the palato-maxillary
suture, but are rather behind its palatine face, and they are about on
a level with the interval between the tenth and eleventh teeth. If the
anterior edge of the palatine bone bounded them, they would be a little
behind the twelfth tooth. The posterior nares, 2-1/8 inches wide, by
7/8ths of an inch long, look altogether downwards, are completely
divided by a bony septum, and have the form of a rhomboid with its
narrowest side posterior. They are surrounded by a strong raised ridge,
incomplete only at the anterior and outer angles of the rhomboid.
Five species of _Jacare_ are enumerated by Natterer--_J. fissipes_, _J.
sclerops_, _J. nigra_, _J. punctulata_, and _J. vallifrons_. They have
met with only in South America.
[1] "Beitrag zur näheren Kenntniss der Sudamerikanischen
Alligatoren," 'Annalen des Wiener Mus.,' Band i.
[2] According to Natterer, the dental formula of _J. nigra_ and
_J. fissipes_ is 18-18/18-18, of _J. sclerops_ 19-19/20-20, of _J.
callifrons_ and _J. punctulata_ 20-20/18-18.
* * * * *
II. In the family of the CROCODILIDÆ the teeth are usually strong and
very unequal in size, and there is always a considerable difference
between the anterior and the posterior teeth. The two anterior
mandibular teeth are received into pits in the premaxilla; but the
canines pass into grooves (which may be converted into fossæ) situated
at the junction of the premaxilla and maxilla. The other mandibular
teeth are received between the maxillary teeth. The symphysis of the
lower jaw does not extend beyond the level of the seventh or the eighth
mandibular tooth. The premaxillo-maxillary suture may be either straight
or strongly convex backwards. The divided vomers do not appear in
the palate. The posterior nares look more or less backwards, and are
transversely elongated. The supra-temporal fossæ are always open, and
the feet are distinctly webbed. The dorsal scutes are not articulated;
and there are no ventral scutes.
Two genera, _Crocodilus_ and _Mecistops_, are distinguishable in this
family.
Genus 4. CROCODILUS.
The teeth are always strong and very unequal, the strongest in the upper
jaw being the tenth. The mandibular symphysis does not extend beyond the
level of the sixth tooth. There are usually six cervical scutes, in two
rows, or forming a rhomb, and separated by a distinct interval from the
tergal scutes. There are 18 or 19 teeth above, and 15 below, on each
side.