450 ORDERS OF MAMMALIA. 



of small size, and are mostly only indicated by small apertures, 

 which the animal has the power of closing when under water. 

 The bones are light and spongy, and beneath the skin is a layer 

 of fat or blubber. The dentition varies ; but teeth of three 

 kinds are always present, in the young animal at any rate. 

 The canines are always long and pointed, and the molars are 

 generally furnished with sharp cutting edges. 



The Seals (Photida) are distinguished by having incisor 

 teeth in both jaws, and by the fact that the canines are not 

 immoderately developed. As regards their distribution in 

 time, the Seals are indicated as occurring in the Miocene and 

 Pliocene Tertiary ; but their remains are by no means as 

 abundant as might have been anticipated from their aquatic 

 habits. Remains of Seals, however, are by no means very rare 

 in Post'Tertiary deposits. 



The Walruses (Trichecidce) are distinguished from the Seals 

 by their enormously -developed upper canines, which grow 

 from persistent pulps, and constitute great pointed tusks. 

 Remains of the Walrus have been found in some of the later 

 Tertiary deposits, but they are merely fragmentary, and are of 

 little importance. 



SECTION II. PLANTIGRADA. The Carnivorous animals be- 

 longing to this section apply the whole or the greater part of 

 the sole of the foot to the ground (fig. 366, A) ; and the por- 

 tion of the sole so employed is destitute of hairs in most 

 instances (the sole is hairy in the Polar Bear). 



The typical family of the Plantigrade Carnivora is that of 

 the Ursida or Bears, in which the entire sole of the foot is 

 applied to the ground in walking. The Ursida are much less 

 purely carnivorous than the majority of the order, and, in 

 accordance with their omnivorous habits, the teeth do not 

 exhibit the typical carnivorous characters. The incisors and 

 canines have the ordinary carnivorous form, but the " carnas- 

 sial" or sectorial molar has a tuberculate crown instead of a 

 sharp cutting edge. The dental formula is 



/3=3 ; c *=*-.** 4=4; ; 2 =? = 42. 

 33 i i 44 33 



The claws are large, strong, and curved, but are not retrac- 

 tile. The tongue is smooth ; the ears small, erect, and rounded ; 

 the tail short ; the nose forms a movable truncated snout ; and 

 the pupil is circular. 



The oldest known member of the Ursida is the Atxphicyon 

 of the Miocene Tertiary. In this genus the molars are tuber- 

 culated, as they are in the true Bears ; but, unlike the latter, 



