MAMMALS. 403 



nivores, and in some the rodents. They belong largely to the eocene, and 

 the United States has furnished the greater number of specimens, Europe 

 having but few. Cope has united these forms with the insectivores and cre- 

 odonts into an order Bunotheria. Esthonyx^ eocene, New Mexico. Tillo- 

 therium* eocene, Wyoming. Calamodon {Stylinodoti) . Stagodon, cretaceous. 



ORDER VI. SIRENIA. 



Thick skinned, naked or sparsely haired, aquatic, placental 

 mammals, with monophyoclont teeth ; with fin-like fore limbs ; 

 hind limbs lacking ; a horizontal caudal fin ; a movable elbow 

 joint ; small brain with few and shallow convolutions ; two pectoral 

 mammae. 



The sirenia contains a few aquatic mammals, which externally 

 resemble the whales in their fusiform bodies, flipper-like fore 

 limbs, absence of hind limbs, and flattened caudal fin. In more 

 important features they are greatly different, and nothing that 

 is known of development or geological history points to their 

 having descended from a common stock. They have the nostrils 

 separate, and opening forward, small eyes with well-developed 

 nictitating membrane, no conch to the ear, no dorsal fin. The 

 paddle-shaped fore limbs have the digits enveloped in the com- 

 mon integument, and only occasionally are nails present. The 

 bones are very dense, and the long bones are without central 

 cavities. Only occasionally are any of the cervical vertebrae 

 anchylosed, and in the recent forms no vertebrae unite to form 

 a sacrum. The anterior caudal vertebrae bear chevron bones. 

 In the skull the chief features are the great development of the 

 zygomatic arch, the reduced nasals, the downward flexure of the 

 jaws in front, and the lower jaw with an ascending ramus. In 

 the fore arm the radius and ulna are usually anchylosed at their 

 extremities ; the digits are always five, and there is no such in- 

 crease in the number of phalanges as occurs in the cetacea. No 

 clavicles are developed. The pelvis is represented by a pair of 

 bones lying at some distance from the vertebral column. 



Incisors and molars alone are present in the recent forms, 

 and in one genus (Rhytina) no teeth occur, at least in the 

 adult. Many fossil species had a more heterodont dentition, 

 and in Halitherium there was a milk dentition not known in 



