THE SIRENIA. 387 



greatly bowed (whence the name of the genus"), so as to 

 be convex outwards and concave inwards. They grow 

 from persistent pulps, and the enamel is absent upon their 

 inner faces. 



The centra of the cervical vertebrae have flattened arti- 

 cular faces. The dorsolumbar vertebrae and the sacrum 

 are not known. The ribs are spongy internally, like those 

 of ordinary Mammals, not compact, as in the Sirenia. 



The scapula has a very large supraspinous fossa, as in 

 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 coitc- 

 sponding bone in the Elephants. 



It is a curious comment upon the pretension to recon- 

 struct 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 Toxodon, no one ventures to predict the characters 

 of its feet, still less to say anything about its internal 

 organization. Even its zoological affinities are extremely 

 doubtful, and it is hard to say whether Toxodon is merely 

 an aben-ant Ungulate, or whether it is the type of a new 

 order. 



III. The SiEENiA. — 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 integu- 

 ment of the caiidal 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 ob- 

 scurely marked, and the forelimbs 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, which are 

 perfectly distinct from one another, are situated consider- 



