105 
cavity is obliterated, but a short, thick, semilunar fold of the lining 
membrane of the aorta, with its concavity twmed towards the end of 
the arch, indicates the place of the former foetal communicating chan- 
nel. The left auricle and ventricle offer no peculiarities. The coats 
of the aortic arch are thickest near the origins of the great vessels 
sent to the head and fore-limbs, especially at the lower part of the 
arch, where they are 3 lines thick. The right subclavian and carotid 
arise close together, but scarcely from a common trunk: the left 
carotid and left subclavian have more obviously separate origins. The 
aorta suddenly diminishes beyond those primary branches, where it 
is connected with the consolidated and contracted ‘ ductus arteriosus.’ 
There is no arrangement, even in the very young Walrus here de- 
scribed, for accommodating the animal to its aquatic habits, by any 
direct intercommunication between the right and left sides of the 
heart. The hepatic veins, in their dilatations, resemble those of the 
ordinary Seals. 
The brain weighed 1 lb. 9 oz. avoir.; its convolutions and struc- 
ture were described. The hip-joint was found to be devoid of the 
ligamentum teres. Drawings were exhibited of the viscera of the 
thorax and abdomen, and the Memoir was terminated by a minute 
account of the dentition, and a disquisition on the homologies of the 
teeth of the Walrus, as elucidated by the state of the dentition im the 
young animal dissected. 
Professor Owen detected the following minute teeth exposed or 
buried in the gum, and adhering to the gum and periosteum of the 
jaws :—two denticles in each premaxillary bone; two denticles in 
each maxillary bone ; together with a deciduous canine, and four den- 
ticles in each ramus of the mandible. He gave the following as the 
formula of the deciduous dentition of the Walrus :— 
- 2—2 1 2—2 
army) C y-? m == 18. 
Of the permanent series, the canine tusks had pushed through the 
gum, aud on laying open the substance of the jaws, the following 
teeth were found in course of formation :—in the premaxillary, the 
successor of the second minute incisor; in the maxillary, the succes- 
sors of the two deciduous molars, together with a third molar. In 
the lower jaw the successor of the second deciduous incisor, the suc- 
cessors of the two deciduous molars, and a third similar permanent 
molar. The germ of the permanent tusk, confined to the upper 
jaw, was 2 inches in length, and, like the germs of the smaller per- 
manent teeth, its base was widely open, and contained a large forma- 
tive pulp. 
In addition to the upper canine tusks, the normal number of per- 
manent and functional teeth in the Walrus is four on each side of 
both jaws; these teeth are simple, short, thick and obtuse, having 
the office of grinders. With respect to their nature and homologies, 
Professor Owen argued that the first, from its position in the pre- 
maxillary bone, was an ‘incisor’; the two following teeth, by their 
position in the maxillary bone, and their relation to the deciduous 
denticles, were ‘premolar’ teeth ; and he also regarded the last of 
the series of four, from its minor degree of development, as belong- 
