mechanical instrument, subservient to the function o f digestion ; we next proceed
to consider the structure and composition o f those cavities o f the skull which gave
lodgment and protection to the organs o f special sense, and endeavour to deduce
from their structure conclusions as to the degree in which the organs were developed,
and the circumstances under which the senses were exercised.
The orbit o f Toxodon forms the anterior boundary o f the zygomatic area;
it is about as distinctly defined as in the Tapir or D ugong, having its osseous rim
le ss complete than in the Hippopotamus, y e t more developed tlian in the Capybara,
Coypiis, and many other Rodentia, in which the orbit is scarcely distinguishable
in the cranium from the small space occupied by the origin o f the temporal
muscle.
The lower boundary o f the orbit in Toxodon is formed by an excavation in
the upper and anterior part o f the zygoma ; the upper boundary b y a strong and
rugged overarching process o f the frontal bone, the posterior angle o f which
(a, PI. I I I .) descends a little way, but leaves a space o f three inches and a h a lf between
it and the opposite angle o f the malar bone below (&, PI. I I . and I II .), the
circumference o f the orbit being completed probably b y ligament in the recent subje
c t. The cavity thus circumscribed is remarkable for the preponderance o f the
vertical over the transverse or longitudinal diameter, and indicates great ex tent o f
motion o f the eyeball in the'vertical direction, such as may be supposed to be well
adapted to the ex ig encies o f an amphibious quadruped. The orbit o f the Capybara,
or Water-hog, makes a near approach to the form ju st described. In the elevation
o f the supra-orbital boundary, and its outward projection in the Toxodon, we perce
ive an approximation to the form o f the orbit in the Hippopotamus, but the size
o f the orbit is relatively larger in the Toxodon, which in this respect manifests
its affinity to the Rodentia.
In that part o f the bony structure o f the auditory apparatus, which is visible on
the exterior o f the cranium, the skull o f the Toxodon presents a character in which
it recedes from the Rodentia. In these, the tympanic portion o f the temporal bone
is remarkably developed, forming a large bulla ossea between the glenoid cavity and
the occiput; and it always remains disunited to the other elements o f the temporal
bone. In the Toxodon the tympanic bone (c, PI. I I .) consists o f a rough compressed
vertical osseous plate, wedged in transversely between the occiput and the
posterior part o f the glenoid cavity. The internal extremity o f this plate points
inwards and forwards, representing the styloid p r o c e ss ; behind this is seen the
petrous bone, which forms a small angular protuberance at the b a sis crariii, and is
le ss developed than in the Hippopotamus. Anterior to the petrous bone are the
orifices o f the Eustachian tube, and carotid c an a l; external to it is the great
foramen laceruin, for the jugular vein and nervus v a gu s; and behind it is the
anterior cohdyloid foramen. T h e foramen auditorium externum is only h a lf an
inch in diameter, and gives pa ssa g e to a long and somewhat tortuous meatus,
which passes inwards and slightly forwards and downwards; its direction being
precisely the same as in the Hippopotamus ; it was accompanied, probably, by as
small an external auricle.
Bu t the indications o f the aquatic habits o f the Toxodon, which are presented
by the osseous parts relating to the sense s o f sight and hearing, are o f minor import
compared with those afforded by the bony boundary o f the nostrils. This
boundary circumscribes a large ovate aperture, the aspect o f whose plane is u p wards,
and a little forwards, as in the Herbivorous Cetaceans, and especially the
Manatee {Trichecus Manatus, Cuv.) In one part o f the bony structure o f the
nasal cavity the Toxodon deviates, however, in a marked degree from the Cetaceous
structure ; I allude to the frontal sinuses, which are exposed by the fracture o f
the upper part o f the skull. (Th ey are shewn in P la te I I I ., and an asterisk is
placed on one o f the narrow canals o f intercommunication between the sinuses and
the nasal passages.) T h e posterior orifice o f the nasal cavity is relatively larger
and wider than in the Herbivorous Cetaceans, and differs both in form and a spect
in consequence o f the greater extent o f the bony palate. T h e Toxodon further
differs from the Manatee and D ugong, in the firm nature o f the connexion o f the
bones o f the head; and it differs from the Hippopotamus in the strong attachment
o f the intermaxillary bones to the maxillaries.
There next remain to be described, as far as the shattered condition o f the
skull will permit, the relative position, extent, and connexions o f the principal bones
composing it.
The occipital hone exhibits a complete confluence o f its basilar, condyloid, and
supra-occipital elements. Th e basilar portion, in connexion with the corresponding
element o f the sphenoid bone, describes a curve whose convexity is downwards.
The condyles are large, extended in the transverse direction, completely
terminal, and a little inclined downwards below the lev el o f the basilar process.
The curve o f the articulating surface describes, in the vertical direction, two-
thirds o f a circle, indicating that the head must have possessed considerable
ex tent o f motion upwards and downwards upon the atlas ; thus, while the
body o f the Toxodon was submerged, the head probably could be raised so as to
form an angle with the neck, and bring the snout to the surface o f the water
without the nece ssity o f any corresponding inflection o f the spine. Indeed, in
the form and position o f the condyles, the Toxodon more nearly resembles the
true Cetacea than any other ex isting mammalia; and it is only with these that
it can be compared in regard to the a spect o f the plane o f the occipital foramen,
and o f the occipital region o f the skull. This is inclined forwards from the occipital
foramen at such an angle, that on viewing the skull from above, not only the