skull o f a Sumatran two-horned Rhinoceros, in the Museum o f the College o f
Surgeons (No. 816), the circumference o f that part o f the surface o f the skull
which supported the posterior horn, and which includes precisely the same part
o f the os frontis, presents the same character, the surface being broken b y numerous
vascular impressions. On the supposition that this character o f the supra-
orbitary arch in the Toxodon might indicate the superincumbency o f a bony case,
I examined the skulls o f two Armadillos, Dasypus P eb a and D a s. Q-cinctus, and
found that in the Da syp u s 6-cinctus, the supra-orbital ridges, wliich are slightly
elevated, to support the cephalic plate, presented, in a minor degree, a corresponding
rugosity. May we venture then to conjecture that the Toxodon was defended
by an ossified integument lik e the Armadillo, or that it was armed with an
epidermic production, analogous to the horn o f the R hinoc eros; or had the rugous
surface in question as little relation with the parts that covered it as the sculptured
surface o f the malar bones in the Cavy ?
After forming the rugged and prominent supra-orbital processes already
described, the frontal bone continues to send backwards a sligh tly elevated
ridge or crista, circumscribing the origin o f the temporal muscles, but the extent
o f this ridge, and the disposition o f the inter-orbital portion o f the frontal bones
cannot be determined in the present mutilated specimen. T h e fractures it has
sustained are not, however, wholly unattended with advantage ; they expose the
structure o f the diploe, w’hich from its coarseness o f texture and thickness, resembles
that o f the Cetaceous crania ; and what is o f still more importance, they
also demonstrate the existenc e and form o f the frontal sinuses.
The cavity o f the nose is extensive, and the remains o f the ossa spongiosa supe-
riora testify that the Toxodon enjoyed the sen se o f smell to a degree equal at
lea st to that o f the Hippopotamus.
The sphenoid bone resembles that o f the Hippopotamus, but it contributes a
larger share to the formation o f the internal pterygoid processes {p , PI. I I .); these
are o f a simple form, and more developed than in the Hippopotamus; they project
outwards to a greater extent, and terminate in a point. The sphenoid also sends off
a short and thick pointed process from the posterior part o f the base o f the internal
pterygoid processes. T h e ala o f the sphenoid does not rise so far into the orbit, nor
does it articulate with the parietal bone, as in the Hippopotamus; but in this part of
its structure, is, the same as in the Rhinoceros. The spheno-palatine foramen is
relatively larger than in the above-named Pachyderms, and is bounded above by
the descending orbital plate o f the frontal bone.
Th e palatal processes o f the palatine bones terminate anteriorly between the
la st molars, and extend backwards for some distance beyond the alveolar proc
e sse s, increasing the extent o f the bony roof o f the mouth poste rior ly: this is a
structure in which the Toxodon deviates both from the Rodents, and Pachyderms,
and resembles the Armadillos among the E d e n ta ta ; ex c ep tin g that the postdental
part o f the bony palate in the Toxodon is suddenly contracted in breadth.
The palato-maxillary suture is in the form o f a chevron, with the angle directed
forwards, as in the Hippopotamus and Capybara, but truncated.
The superior maxillary bones ( f , PI. I I .) are united posteriorly to the malar, as
above described: they ascend and jo in the frontal and nasal b o n e s : their outer surface
is almost vertical, smooth, and sligh tly u nd u la ting ; perforated at its posterior
part by the ant-orbital foramen, and joined anteriorly to the intermaxillaries b y a
suture running in the sigmoid direction (as shewn in PL II.) from the middle o f the
nasal cavity, to within four inches o f the anterior boundary o f the upper jaw.
We have, in the position and ex ten t o f this suture, and the absence o f tusks and
their large prominent sockets, a m ost important difference between the Toxodon and
Hippopotamus. The ch ie f peculiarity in the maxillary bones, obtains in the
arched form o f the alveolar processes, corresponding to the shape and position of
the grinders above described, and which are peculiar among known mammalia to
the present genus. Th e palatal surface o f the maxillary bones is obliquely perforated
by two large foramina, from which two deep longitudinal grooves extend
forwards, and are gradually lo s t ; we find the posterior palatine foramina represented
b y similar grooves and foramina in the Capybara.
The intermaxillary bones ( d , P is. I I . and I I I .) , though large, are relatively o f
le ss ex tent than in the Rodents generally. Th e nasal processes do not reach the
frontal bone, but are limited to the anterior h a lf o f the nasal boundary; approaching
in this respect to the Herbivorous Cetacea. In the outward expansion o f their
anterior extremities, the intermaxillaries resemble those of the Hippopotamus, in
which, however, this character is more strongly marked. T h e intermaxillaries
in the Hippopotamus are also much le ss firmly united to the maxillary
bones than in the Toxodon, and are consequently commonly lost in the fossil
crania. On the palatal surface o f the intermaxillary bones there are two grooves
which diverge forwards from the line o f the suture ; and anteriorly to these grooves
there are the two large anterior palatine foramina. Th e maxillo-intermaxillary
sutures on the palate converge as they extend backwards to a p o in t; there appears
to have been a fissure left between this suture and the mesial suture o f the intermaxillaries
; in which structure the Toxodon resembles the Hippopotamus.
After summing up the different affinities, or indications o f affinity, which are
deducible from the cranium o f this most curious and interesting fossil mammal,
we are led to the conclusion, assuming it to have had extremities cased in hoofs,
that it is referrible to the Order Pachydermata. B u t the structure, form, and
kind o f teeth in the upper jaw, prove, indisputably, that the gigantic Toxodon
was intimately related to the Rodent Order. From the characters o f this order,
as afforded b y the ex isting spe cie s, the Toxodon, however, differs in the relative