TOXODONTIA
The toxodonts of the Deseado are much more varied than those of
the Santa Cruz, and less so than those of the Casamayor; the teeth
less hypsodont than in the Santa Cruz, and more hypsodont than in
the Casamayor; are smaller than those of the Santa Cruz, and larger
than those from the Casamayor. It is a group of heavy, short-limbed,
nonadaptive animals, which, as time and competition progressed,
gradually diminished in numbers and variety.
The ancestral type must be sought in some such a form as
_Henricosbornia_, where the upper molars are brachydont, have the four
primary cusps distinct, and the connecting crests of small size, and a
cingulum moderately developed on the front and rear sides. Progress is
in the line of enlarging the crests, so that, in the later forms, the
two external cusps are united to make a wall; and the anterior external
and the anterior internal cusps are united into the large anterior
lobe; while the posterior external and the posterior internal cusps
unite to make the posterior lobe. These may remain relatively simple as
in _Rhynchippidae_[14]; or with this simple arrangement of the cusps,
the cingulum may be developed into a platform around the anterior,
inner, and posterior sides of the molars, as in the _Isotemnidae_; or,
with relatively simple molars, the incisors may be specialized into
caniniform-like teeth as in the _Leontinidae_; or secondary processes
(or cristae) may develop from the wall, making the complicated teeth
characteristic of _Nesodontidae_.
[Footnote 14: I have abandoned the family term _Notohippidae_,
as the genus used as a basis is very little known, and the forms
Ameghino assigns to the family, to my mind, mostly belong with the
_Nesodontidae_.]
For convenience in discussing the modifications of the toxodont tooth,
I have, throughout, used the nomenclature illustrated in fig. 52,
taking one of the most complicated to show the ultimate development.
In the upper tooth there is, first, the external wall, from which
springs the anterior lobe, always the larger lobe, and composed
of the protocone and paracone of Osborn. In the rear is a smaller
narrower posterior lobe, composed of the hypocone, the metacone, and
the metaconule of Osborn. Between these is a large basin, which may
be subdivided by two cristae into secondary bays, referred to as bays
1, 2 and 3, while the cristae are in the same way referred to as
cristae 1 and 2. In some genera, the cristae are entirely wanting, in
others incipient. When fully developed, they are most marked in young
individuals and, as the tooth is worn, appear progressively shorter.
Behind the posterior lobe, there is a variable bay, number 4 which is
bounded behind by crista 3, which is apparently a development of the
posterior cingulum. This last crista and bay may or may not be present.
[Illustration: Fig. 52. _A_, upper molars of Coresodon; _B_, lower
molars, somewhat worn—natural size.]
The lower molars of toxodonts are all on the same plan, each tooth
being composed of two crescents, the anterior and posterior. The
ends of these crescents are referred to as the anterior, median and
posterior horns. The bay in the anterior crescent is simple and usually
disappears with the wear of the tooth without making a pit. In the
centre of the posterior crescent is the pillar or posterior tubercle
which Scott has found to be characteristic of these South American
Ungulates. It is, to my mind, the same as the mesostylid of the Fayum
hyracoids. Between the pillar and the median horn, I find a narrow
vertical ridge, which I have termed the septum; and which tends to
unite with the pillar inclosing a small bay, usually seen in worn teeth
as a pit. The bay between the septum and the median horn is designated
bay 2, and this quite generally appears in a worn tooth as a pit
(2). The bay between the septum and pillar is designated bay 3, and
is usually seen as a tiny pit, which however does not extend as deep
into the crown as the other pits and is usually lost when the tooth
is about half worn off. The bay between the pillar and the posterior
horn is numbered 4, and is usually open, though in a worn tooth it also
may appear as a pit. The effect of wear is shown by comparing B and C
in fig. 52, the latter being the same tooth sectioned a little below
the middle. I find in studying a lower molar of Coresodon that bay 3
becomes a pit after some 6 mm. are worn off, while bays 2 and 4 remain
open until some 10 mm. are worn off when they also become pits. Pit 3
will disappear when 12 mm. are worn off, but pits 2 and 4 run to the
base of the crown.
The various genera of the _Toxodontia_ in the Deseado I would divide
into four families as follows:
=Rhynchippidae=: molars brachydont, secondary cristae lacking or
little developed, none of the incisors caniniform, limbs slender, feet
digitigrade, digits 3-3.
=Leontinidae=: molars brachydont, secondary cristae lacking or little
developed, upper inc. 2 and lower inc. 3 developed into caniniform
teeth, limbs heavy, feet digitigrade, digits 3-3 (according to Gaudry).
=Isotemnidae=: molars brachydont, secondary cristae more or less
developed, crowns contracted at the top, congulum more or less
developed into a platform, skeleton unknown.
=Nesodontidae=: molars hypsodont, secondary cristae highly developed,
upper inc. 2 and lower inc. 3 caniniform, limbs heavy, feet
digitigrade, digits 3-3.
=Rhynchippidae=
This family name is used for the three genera _Rhynchippus,
Morphippus_, and _Eurygeniops_, which made up a part of Ameghino’s
family, _Notohippidae_. These forms I find much simpler than
_Coresodon_, _Interhippus_, _Stilhippus_, and _Nesohippus_, which,
by their molars, should be associated with _Nesodontidae_, unless it
should prove that they did not have the incisors enlarged to caniniform
teeth, in which case another disposition will have to be made of them.
Ameghino places the _Rhynchippidae_ among his _Hippoidea_, leading
to horses, but we found a nearly complete skeleton of _Rhynchippus
equinus_ which in all particulars is typically toxodont. In the Deseado
we found fourteen specimens belonging to this family, and strangely
enough they were all _Rhynchippus_, and all of the species _R. equinus_.
This family is distinguished by the brachydont, or nearly brachydont,
molar teeth, being relatively simple, and the secondary cristae not
being developed. The large basin in the upper premolars and molars is,
therefore, not subdivided, but is deep, and rather narrow, usually
appearing as an oblique pit in the centre of the crown. There is no
enlargement of the incisors to make caniniform teeth. Both the upper
incisors and the canine have in the crown a longitudinal groove, which
on wear becomes a pit, and being shallow may disappear entirely. The
lower teeth are those typical of all toxodonts. The feet are tridactyl,
and compact.
The following three genera may be distinguished:
RHYNCHIPPUS MORPHIPPUS EURYGENIOPS
3 1 4 3 3 1 4 3 3 1 4 3
Formula ------- ------- -------
3 1 4 3 3 1 4 3 3 1 4 3
Skull moderately long short muzzle, short heavy
muzzle with slight muzzle,
constriction with marked
behind canines constriction
behind canines
Upper incisors groove or pit groove or pit groove or pit
Lower incisors cingulum on the no cingulum no cingulum
inner face
Upper premolars cingulum on ant. int. cingulum int. cingulum on
corner on ant. corner ant. int. corner
Upper molars basin deep basin shallow basin deep, with
incipient
secondary bays
Lower molars 4 bays bays 1 and 2 bays 1 and 2
only only
=Rhynchippus= Ameghino
Rhynchippus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 462.
The teeth of both jaws are rooted, but tend to be hypsodont. The
elongated incisors of the upper series are characterized by the
presence of a longitudinal furrow in the top of each tooth, which,
with wear, becomes a pit, and, as it is shallow, disappears in old
individuals. This is the only suggestion of a horse character in the
genus, but the pit in a horse’s incisor is a late development, and here
it is also probably a specialization due to eating grass. Incisor 1 is
the largest and they decrease in size toward either side. The canine is
small, and is also marked by having a furrow in the crown, but in this
case it is transverse to the long axis of the jaw.
The premolars are peculiar in having on the anterior internal corner
a highly developed cingulum, which so builds out the tooth that it is
usually wide and is rectangular in outline. As this cingulum rises, it
incloses a bay on the ant. int. corner of the tooth, which, with wear,
becomes first a bay, then a pit, and lastly may disappear entirely
in old age. On each premolar the anterior and posterior lobes are
developed, inclosing between them an elongated basin, which with wear
becomes a long narrow pit. On the molars, the cingulum on the ant. int.
corner is wanting entirely. The external anterior corner of the tooth,
however, is prolonged, so that the crown has a rhomboidal outline. The
crown is made up in the typical manner of the wall, the anterior, and
the posterior lobes, which inclose between them an elongated basin,
which, as in the premolars, becomes, on wear, a pit extending obliquely
across the tooth.
The lower incisors have no furrows in the crowns, but in this genus
there is a small cingulum on the inner side just above the base of
the enamel. The lower canine is incisiform, and also has the basal
cingulum. Each of the premolars has, on the external side, a median
vertical groove, beginning at the base of the enamel, and widening
toward the top. This is progressive if less marked from pm. 1 to pm.
4. The premolars and molars consist essentially of two crescents, the
shorter anterior, and the posterior which is about twice as long as the
anterior. The details are as described on page 96, and seen in figure
55.
The skull is of moderate height, nearly flat on top with wide zygomatic
arches. The sagittal crest is moderately high, and slightly convex in
the antero-posterior direction. The occipital region is overhanging
and topped by short lambdoidal crests, which, extending to either
side, unite with the zygomatic arches. The nasals are large, roughly
rectangular, and slightly constricted just in front of the middle. The
frontals are short, and project over the orbits in strong processes.
The maxilla is large, bounding the front of the orbit, and extending
backward in a strong zygomatic process which makes nearly half of the
arch. The jugal, while stout, is short, and reaches from the long
zygomatic process of the maxilla to the short one of the squamosum.
The lachrymal bone is tiny, with a low tubercle, just below which is
situated the lachrymal duct, just on the margin of the orbit. Just
behind the zygomatic arch, the squamosum is inflated and contains
a large hollow chamber, as is typical among toxodonts. The mastoid
bullae, while relatively small, are swollen into a globular form, and
have a large hollow chamber. The palate extends back to just behind the
last molar, a feature distinguishing this genus from _Morphippus_.
Of the vertebral column, twenty-six vertebra are preserved (a few being
represented by neural arches only). The atlas and axis are unknown.
Five cervicals are present, each with a short, slightly opisthocoelus
centrum, and with low weak spines. The foramena for the vertebra artery
are usually large. Cervical 3 has a rather slender transverse process,
projecting down—and backward. On cervicals 4, 5 and 6, these lateral
processes are enlarged into broad lamellae, which reach their maximum
of size on the sixth. Cervical 7 has no lamella, simply a slender
transverse process. These transverse processes are strikingly like
those of Nesodon. The thoracic vertebrae (of which I have complete
or in parts 15) have moderately high spines, which resemble those of
Adinotherium, not only in the general build, but also in the presence
of a foramen for the exit of spinal nerves through each neural arch.
These foramena can not be referred to as adaptations, but are special
features indicating close relationship with the _Nesodontidae_. Six
lumbar vertebra are present, each having broad depressed centra, and
short wide spines. The rest of the column is unknown.
The distal portion of the humerus is preserved, showing the trochlea to
be relatively narrow, with a prominent internal phlange for the ulna.
The epicondyles are both small. The supratrochlear fossa is moderately
deep, the anconeal fossa very deep, a large perforation connecting the
two. Of the ulna, only the distal end is preserved, and it is marked
by a prominent styloid process, ending in the facet for the pyramidal,
this facet continuing uninterruptedly into that for the pisiform.
The two ends of the radius are preserved but its length can only be
conjectured. The proximal end has a large facet for the humerus; the
distal end two facets, for the scaphoid and luna respectively, the
two being almost continuous, except as the outline of the shallow
depressions is constricted near the middle.
The carpus is preserved _in toto_ and is decidedly weak for an animal
of this size, though no more so than is the case of Nesodon and
Adinotherium, with which forms the arrangement of the bones agrees
in almost every detail. The scaphoid has a broad facet on the upper
side for the radius, on the ulna side a narrow band-like facet for the
luna, and distally facets for the trapezoid and the magnum, none for
the trapezium. The luna is a larger bone with a broad radial facet
on the upper side, a narrow facet for the scaphoid on the radial
side, a larger one for the pyramidal on the ulnal side, and two broad
facets for the magnum and unciform on the lower side. The pyramidal
is a low, flattened bone, with a cup-like facet for the ulna on the
upper surface, an elongated flat facet for the pisiform on the palmar
surface, and below a broad facet occupying the entire lower side for
the unciform. The pisiform is shaped like a tiny calcaneum, and, beside
the facet for the pyramidal, has a broad contact on the styliform
process of the ulna. The trapezium is a flattened nodular bone, resting
against the side of the upper end of Mc. II, for which it has a
flattened contact surface, but it does not properly articulate with any
of the carpals. The trapezoid is a small bone, cuboidal in shape, with
the proximal facet for the scaphoid rounded, and with a narrow facet
for the support of Mc. II on the distal end. The magnum is a larger
bone, articulating proximally with the scaphoid and luna, on the ulnal
side with the unciform and distally supporting Mc. III. The unciform is
the largest of the carpal bones, articulates proximally on the luna and
pyramidal, on the radial side with the magnum, and distally carries Mc.
IV, while on the ulnal side there is a facet for the modular vestige of
Mc. V.
The metacarpals are longer than those of Adinotherium, and are much
more closely pressed together, making a narrower, firmer foot. Three
metacarpals are present (beside the modular vestige of Mc. V.). Mc.
II and Mc. IV are slightly shorter than Mc. III, but not materially
weaker, so that all three would reach the ground when the animal was
standing. Though closely packed, the metacarpals are not grown together
at any point. Distally each has a narrow trochlea, which carries a
median crest on the palmar side only. Under the distal end of each
metacarpal, there is a pair of small sesamoid bones.
All the phalanges are very short and weak, with the articular surfaces
cut under obliquely, suggesting that the toes are bent upward. Distally
each toe ends in a slender cleft ungual phalanx, suggesting a claw-like
hoof. _Rhynchippus_ clearly walked in a semidigitigrade manner, the
weight coming principally on the metapodials, the foot as a whole
resembling that of a dog.
The pelvis is unidentified. Though the bones are of about the same
weight, the hind limb is longer than that of Adinotherium. The femur
has a rounded head on a short but distinct neck, with the pit for the
round ligament on the posterior margin of the head. The narrow digital
fossa is deep. The greater trochanter is rugose and strong, but does
not rise quite to the height of the head. The lesser trochanter makes
a strong shelf-like process well below the head, while the third
trochanter is a prominent process about the middle of the shaft, on the
posterior side. The shaft is broad and flattened above, but narrows and
becomes circular in section below. The condyles are relatively small,
the inner being the smaller and more convex, while the outer is broader
and less convex.
The tibia and fibula are a little longer than the femur, fused
proximally, free distally, as in toxodonts. The proximal end of the
tibia is too weathered to permit detailed description. However the
upper end is wide and flattened from front to back. Distally the bone
narrows until the lower part of the shaft is circular in section, the
distal end enlarging again in the neighborhood of the articulation. The
fibula has a broad facet for the inner side of the astragulus, and on
the distal end a flattened slightly concave facet for the calcaneum.
The calcaneum is longer than in _Nesodontidae_. It is, however, heavy
and stout, the tuber broadening slightly toward the free end, on the
plantar side of which there is a tendinous sulcus as in Nesodon. The
fibular facet is wide, rectangular, and convex. Of the astragular
facets, the sustentacular extends well back onto the tuber, and the
ectal is the usual ovoid surface. Distally there is a broad slightly
concave facet for the cuboid, and external to this a narrow surface
for contact on the navicular. Except in length, this bone is very like
that of Adinotherium. The astragulus and rest of the tarsal bones are
wanting. Parts of the metatarsals and a phalanx indicate that the hind
foot is of the same tridactyl type as the front, differing only in that
the median digit seems to be relatively a little heavier.
Ameghino described three species of _Rhynchippus, R. equinus, R.
pumulis_, and _R. medianus_.
=Rhynchippus equinus= Ameghino
R. equinus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 463.
This species is the dominant one in the Deseado from the Chico del
Chubut, west of Puerto Pisser, no less than fourteen individuals
being represented in our collection, three by skulls, and one by the
major part of a skeleton which was found associated with the skull of
_Leontinia_, but was determined as belonging to this species, by the
duplication of the radius with a specimen having the radius and lower
jaws associated. The description of the generic characters is largely
based on this skeleton. The three species are differentiated largely by
their size, _R. equinus_ being the largest, but as compared with _R.
pumulis_ it is not only larger but much heavier built.
[Illustration: Fig. 53. Dorsal view of skull—½ natural size.]
The skull has been described under the generic discussion. In young
individuals, the furrow in the incisors and canine is marked as a
groove, later as a pit, and still later is lacking altogether, as is
the case in the specimen figured, for all three of my more complete
specimens are old individuals, as is also Ameghino’s type. All the
incisors show the cingulum near the base of the enamel. On account
of the development of the cingulum on the inner anterior corner of
the premolars, these teeth are broadened out and overhang the palate
in a marked degree, and are also much wider than long. Molar 1 is
intermediate in character between the premolars and the succeeding
molars, being only slightly rhomboidal in outline, while in the last
two molars the anterior external corner is markedly prolonged.
[Illustration: Fig. 54. Left upper dentition, old individual—½ natural
size.]
[Illustration: Fig. 55. Right lower dentition of R. equinus; _A_, of a
young individual; _B_, of an old individual—½ natural size.]
In different individuals, the lower jaw varies greatly in height, but
this seems to me to be individual and sex variation. The three incisors
and the canine are subequal in size, closely crowded, and each with a
small cingulum near the base of the enamel.
MEASUREMENTS OF SKULL, SPECIMEN 3191
Skull, length from front of nasal to back of lambdoidal crest 211 mm.
Skull, width across zygomatic arches 172 mm.
Skull, width across postorbital processes 78 mm.
Skull, height above molar 2 84 mm.
Dentition, from upper inc. 1 to molar 3 140 mm.
Dentition, from lower inc. 1 to molar 3 136 mm.
Mandible, height under molar 2 35 mm.
Mandible, height under the articulation 118 mm.
The atlas and axis are wanting, and the characteristics of the other
cervicals have been given under the generic description.
MEASUREMENTS, SPECIMEN NO. 3291.
Cervical 3, length of centrum 18 mm.
Cervical 5, length of centrum 18 mm.
Cervical 6, length of centrum 21 mm.
Cervical 6, height from base of centrum to top of spine 74 mm.
Cervical 7, length of centrum 21 mm.
Cervical 7, height from base of centrum to top of spine 55 mm.
[Illustration: Fig. 56. Cervicals. 5, 6 and 7—½ natural size.]
[Illustration: Fig. 57. Dorsal 6 and 7—½ natural size.]
The first three or four of the thoracic vertebrae are represented only
by their neural arches and spines. There were at least fifteen in the
series, for I have that number represented, but more probably the
number was sixteen as in the case in _Adinotherium_. Typical vertebrae
measure:
SPECIMEN 3291
Thoracic 3, length of centrum 21 mm.
Thoracic 3, height from base of centrum to spine 56 mm.
Thoracic 6, length of centrum 22 mm.
Thoracic 6, height from base of centrum to spine 79 mm.
Thoracic 14, length of centrum 26 mm.
Thoracic 14, height from base of centrum to spine 65 mm.
In my series, there are six lumbars, which is one more than is
credited to _Adinotherium_, though in that genus the number has not
been definitely fixed. Typical lumbars measure as follows:
Lumbar 2, length of centrum 29 mm.
Lumbar 2, height from base of centrum to spine 65 mm.
Lumbar 4, length of centrum 31 mm.
Lumbar 4, height from base of centrum to spine 57 mm.
There is nothing to represent the sacrum or caudals.
Only the lower half of the humerus is preserved and that with specimen
3191, and it measures:
Humerus, diameter of shaft 22 mm.
Humerus, greatest width of distal end 46 mm.
Humerus, width of trochlea 36 mm.
[Illustration: Fig. 58. Humerus—½ natural size.]
[Illustration: Fig. 59. Right front foot—½ natural size.]
The distal ends of the radius and ulna are preserved in specimen 3291,
as they were found in association with the carpus.
Radius, diameter of the distal end 29 mm.
Ulna, diameter just above styloid process 21 mm.
Ulna, diameter of styloid process 11 mm.
The carpus is carefully drawn, from which the various measurements
may be obtained. There is a tendency for the two rows of carpals to
alternate, but this is not advanced to any considerable degree. The
trapezium is entirely isolated from the other carpals, and lies as a
flattened scale, on the side of the upper end of Mc. II.
The metacarpals are closely crowded together, making a compact foot
with very little freedom of motion in its upper part. The three carpals
are of nearly equal length, though the third is slightly heavier and
longer than the others, but there is no tendency toward a further
reduction of the toes.
[Illustration: Fig. 60. Digit 4 of Rhynchippus from the side—½ natural
size.]
[Illustration: Fig. 61. Pair of sesamoids under Mc. IV—½ natural size.]
Metacarpus II, length 67 mm.
Metacarpus III, length 74 mm.
Metacarpus IV, length 69 mm.
Under each metacarpal are two small sesamoid bones which lie either
side of the low crest of the metacarpus. The toes are all short, with
flattened articular ends, which are cut under in a very oblique manner.
The second phalanx is much shorter than the others, while the distal,
or ungual phalanx, is the longest and highest of the three. Each ungual
phalanx is cleft by a deep narrow notch, much more suggestive of a claw
than a hoof. The phalanges of all the toes are subequal in size, so
that the measurements of the middle digit are given.
First phalanx of digit III, length 12 mm.
Second phalanx of digit III, length 8 mm.
Third phalanx of digit III, length 16 mm.
As preserved, the femur is crushed, and the distal end of the rotular
trochlea is weathered off, but all the other characters are well
preserved. The femur is slender, with a small rounded head. The greater
trochanter is heavy but does not project above the head, the lesser is
small but well marked; and the third is usually far down the shaft. Of
the two condyles the inner is the smaller and more convex.
[Illustration: Fig. 62. Right femur posterior side—½ natural size.]
[Illustration: Fig. 63. Right tibia and fibula posterior side—½ natural
size.]
[Illustration: Fig. 64. Calcaneum—½ natural size.]
SPECIMEN 3291
Femur, greatest length 202 mm.
Femur, diameter of head 26 mm.
Femur, width across head and greater trochanter 62 mm.
Femur, width of internal condyle 16 mm.
Femur, width of external condyle 24 mm.
The tibia is much flattened at the upper end and tapers to nearly
circular in section in the distal portion of the shaft. It is fused
proximally to the fibula, but free distally.
Tibia, length 222 mm.
Tibia, greatest width proximally 51 mm.
Tibia, least diameter of the shaft 24 mm.
Tibia, diameter of distal articular end 23 mm.
The fibula is a very slender bone, with the distal end swollen and
heavy. As it rises from the carpus it is so twisted that it unites with
the upper end of the tibia almost on the posterior surface.
Fibula, diameter of the articular end 17 mm.
Fibula, least diameter of the shaft 10 mm.
The calcaneum is of moderate length, and very stout, resembling that of
Adinotherium, except that it is longer.
Calcaneum, length 64 mm.
Calcaneum, width 28 mm.
Of the hind foot there is preserved only the distal portions of two
metatarsals, which are about the same size and character as those of
the front foot, and a phalanx of the first row, also similar to the
same one of the front foot.
Figure 65 gives a restoration of the animal based on the bones
described in the foregoing pages. The animal in all features turns out
a typical toxodont, adapted, by the cropping teeth, and the broad-faced
premolars and molars, for a grazing animal, but its advancement in
adapting itself to feeding on grass has not proceeded very far, as is
indicated by the shortness of the molars. The legs are longer than
in the other families of the toxodonts which would signify that it
had developed some speed, but the feet have progressed toward the
modification of the hoofs into claws, indicating a foot more like that
of a dog, in which the weight is not carried on the ungual phalanges,
but rather on the ball of the foot, or bases of the metapodials.
[Illustration: Fig. 65. (Half tone.) Restoration of Rhynchippus
equinus, Ameghino—⅙ natural size.]
I should not feel that this group was the ancestral one to later groups
of toxodonts, but it seems rather to represent a line which terminates
in the Deseado or very little later, not having run up into the
Santa Cruz. The line of ancestry for the toxodonts is rather through
_Leontinidae_.
=Rhynchippus pumulis= Ameghino
R. pumulis Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 464.
[Illustration: Fig. 66. R. pumulis—½ natural size; _A_, top of skull;
_B_, upper dentition, after Ameghino.]
We found no specimens of this species, but Ameghino has described a
complete skull, a figure of which is reproduced here. It indicates a
smaller lighter built animal, differing from _R. equinus_ not only
in small size, but also in having a relatively longer and narrower
head. The individual is a rather old one, so that the pits in inc.
1 and 2 have disappeared, as is also the case with the cingulum on
the ant. int. corners of the premolars. Ameghino gives the following
measurements in his description.
Skull, length over all 155 mm.
Upper dentition, from inc. 1 to m. 3 80 mm.
=Rhynchippus medianus= Ameghino
R. medianus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 375.
This third species is intermediate in size between the two foregoing.
No figure is given, but the following measurements give the size:
Upper molar 2, length, 17 mm., width 11 mm.
Length of lower molars 1 to 3 40 mm.
Height of mandible under m. 2 24 mm.
These figures would indicate a form about 15% larger than _R. pumulis_,
and 35% smaller than _R. equinus_.
=Morphippus= Ameghino
Morphippus, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459.
This genus is very similar to _Rhynchippus_, with the same dental
formula, the same grooves in the upper incisors, and the same pattern
of the premolars and molars. It differs, however, in the lower
incisors having no cingulum at their base, in the upper molars having
a shallower basin, and in bays 3 and 4 being absent from the lower
molars. These features simply indicate a slightly less advanced
specialization, less hypsodont teeth. I do not think that the bays are
any of them lacking in unworn teeth, but in a less hypsodont tooth,
with the pits extending a less distance into the crown, all indication
of the bays disappears early.
_M. imbricatus_ is described as the type species, and four others have
been described, all equal in size to _M. imbricatus_, and distinguished
by the teeth being slightly more compressed, by the external cleft
of the lower molars being deeper, or by variations in the pits. All
these features I consider to be either age characters or individual
variations, so that all five species of this genus are lumped under _M.
imbricatus_.
=Morphippus imbricatus= Ameghino
M. imbricatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 459.
M. hypsolodus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 461.
M. complicatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 461.
M. fraternus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16,
p. 374.
M. quadrilobus Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16,
p. 374.
The general characters of this species are given under the generic
description and I will here give only Ameghino’s measurements which go
with the figure:
Skull, length over 210 mm.
Skull, length of the palate 120 mm.
Upper dentition, length the inc. 1 to m. 3 120 mm.
Diameter of the palate opposite inc. 3 37 mm.
Diameter of the palate opposite m. 3 75 mm.
Height of mandible under m. 1 33 mm.
[Illustration: Fig. 67. M. imbricatus—½ natural size; _A_, upper
dentition; _B_, lower dentition after Ameghino.]
[Illustration: Fig. 68. E. latirostris, palatal view, after a
photograph of the type—½ natural size; the cross hatched area
represents matrix not yet removed.]
=Eurygeniops= Ameghino
Eurygenium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 655.
Eurygeniops Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 464.
The name first given this genus was found to be preoccupied, and
therefore changed. It is a clear cut genus, differing from the others
in the family in the expansion of the front of the muzzle, and by the
heavy broad character of the skull.
=Eurygeniops latirostris= Ameghino
E. latirostris Amegh., loc. cited above.
This is the type species and is based on a muzzle which has never been
figured, but which I figure, the drawing being made from a photograph
taken by Professor Scott and kindly furnished me. The characters of the
species are those of the genus, with the following measurements for
specific determination, quoted from Ameghino:
Palate, length 130 mm.
Palate, width between incisors 3 41 mm.
Palate, width between premolars 2 33 mm.
Palate, width between molars 3 56 mm.
Upper dentition, length from pm. R. to m. 3 82 mm.
Upper premolar 4, length 11 mm.
Upper premolar 4, width 19 mm.
=Eurygeniops normalis= Ameghino
E. normalis Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 466.
This second species is described as being much smaller than the
preceding, the length from pm. 4 to molar 3 being 65 mm.