330 THE ANATOMY OF VERTEBKATED ANIMALS. 



Tapirus. There is no acromion, and the coronoid is very 

 small. The humerus and the ulna are very massive, but the 

 rest of the fore-limb is unknown. The femur is devoid of any 

 third trochanter, and, like the tibia and astragalus, presents a 

 good many points of resemblance to the corresponding bone 

 in the Elephants. 



It is a curious comment upon the pretension to reconstruct 

 animals from mere fragments of their bones and teeth which 

 some have put forward, that, although we know the skull, the 

 dentition, and the most important of the limb-bones of Toxo- 

 don, no one ventures to predict the characters of its feet, still 

 less to say any thing about its internal organization. Even its 

 zoological affinities are extremely doubtful, and it is hard to 

 say whether Toxodon is merely an aberrant Ungulate, or 

 whether it is the tj'pe of a new order. 



III. The SiRENiA. — As has been already said, nothing is 

 known of the placentation of this small but important group 

 of Mammalia, all the existing forms of which are aquatic in 

 their habits, frequenting great rivers and their estuaries ; and 

 are devoid of hind-limbs, while the integument of the caudal 

 end of the body is produced into a flattened horizontal fin. 

 No dorsal fin is ever present. The demarcation between the 

 head and neck is but obscurely marked, and the fore-limbs are 

 converted into paddles, upon which only rudimentary nails are 

 developed. Scanty bristles cover the surface of the body. 

 The snout is fleshy and tumid, and the valvular nostrils, Avhich 

 are perfectly distinct from one another, are situated consider- 

 ably above its termination. There is a well-developed third 

 eyelid, the pinna of the ear is absent, and the mammae are 

 thoracic ; a circumstance which has probably not a little con- 

 tributed to the origination of the myths respecting the exist- 

 ence of mermaids. 



The Sirenia were formerly united with the Whales and 

 Porpoises as Getacea lierhivora. But their organization dif- 

 fers from that of the true Cetaceans in almost every particular, 

 while they are closeh' allied with the Utigulata. 



The cervical vertebrae are reduced to six in one genus — 

 Manatus. The bodies of these vertebrae are always com- 

 pressed from before backward, but they are never all an- 

 chylosed together (it is rare for any of them to be thus 

 united), and the second has a distinct odontoid process. The 

 dorsal vertebrae have broad and depressed spines, and may be 

 as many as seventeen or eighteen in number, while there are 



