THE PHOCID^. . 361 



jaw. In the mandible there are no incisors, but a single short 

 canine is followed by three, similar, simple teeth, and by one 

 ctlier, which is caducous. 



The dental formula is therefore i. ^ c. ^^ p. m. ni. ^ 



3. The I'hocidoB, or ordinary Seals. — The pinna is alto- 

 gether absent. The hind-limbs are permanently stretched 

 out, parallel with the tail ; and, consequently, they are unable 

 to support the body, or assist in locomotion on land. 



The space between the orbits is extremely narrow, and 

 supra-orbital processes are absent. The bulla tympani is very 

 large and thick-walled ; and the middle are much shorter than 

 the outer digits of the pes. 



The common Seal {Phoca vitulina) is a native and acces- 

 sible member of this group. It has a rounded head and a 

 neck which is well mai-ked, though shorter in proportion than 

 that of the Eared-seals. The nasal apertures are slit-like and 

 can be closed at will, the eyes large and brilliant, and the 

 auditory apertures small and devoid of a pinna. The limbs 

 are large, and their distal longer than their proximal divis- 

 ions. The fore-limb is buried beyond the elbow in the com- 

 mon integument, but the flexible wrist allows the weight of 

 the body to be supported by the palmar surface of the manus. 

 The hind-limbs, on the contrary, are permanently extended 

 and turned backward parallel with the tail, which lies between 

 them, and with which they form a sort of terminal fin. When 

 the Seal swims, in fact, the fore-limbs are applied against the 

 sides of the thorax, and, the hinder moiety of the body being 

 very flexible, the conjoined hind-limbs and tail are put to the 

 same use as the caudal fin of a Cetacean. The Seal has twenty 

 dorso-lumbar vertebrse, of which five are lumbar. There are 

 four sacral vertebras, but only one of these unites with the ilia. 

 Eleven vertebrae enter into the formation of the short tail. 

 There are ten true ribs and nine sternebrse, the manubrium 

 being prolonged forward into a long cartilaginous process. 



The brain-case is smooth, rounded, and spacious, but the 

 cranium narrows rapidly in the interorbital region. Its floor 

 is remarkably flattened from above downward and very thin, 

 the broad basi-occipital sometimes presenting a perforation in 

 the dry skull. The falx is partially, and the tentorium is wholly, 

 ossified. The occipital segment is very large, and the supra- 

 occipital advances between the parietals, but does not separate 

 them completely. The alisphenoids are small and almost hori 

 16 



