388 THE ANATOMY OF VERTEBEATED ANIMALS. 



ably above its termination. There is a well-developed 

 third eyeHd, the pinna of the ear is absent, and the 

 mamnise are thoracic ; a circnmstance which has pi'obably 

 not a little contributed to the origination of the myths 

 respecting the existence of mermaids. 



The Sirenia were formerly united with the Whales and 

 Porpoises as Cetacea herbivora. Bxit their organization 

 differs from that of the true Cetaceans in almost every 

 particular, while they are closely allied with the Ungulata. 



The cei-vical vertebrae are reduced to six in one geniis — 

 Manatus. The bodies of these vertebrae are always com- 

 pressed from before backwards, but they are never all 

 ankylosed 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 not more than three lumbar vertebrae ; and the 

 hindermost of these even is probably to be regarded as 

 sacral. There are twenty or more caudal vertebrae, the 

 terminal ones being not polygonal, but depressed, with 

 well-developed pi-ocesses. 



The zygopophyses of successive vertebrae articulate to- 

 gether in the dorsal region ; but, in the lumbar and caudal 

 regions, the postzygapophyses disappear and the prezygo- 

 pophyses are small, and neither overlap, nor embrace, the 

 spine of the antecedent vertebra. The posterior moiety 

 of the spine thus acquu'es considerable flexibility. There 

 is no ti-ue sacrum, the vertebra called " sacral " being only 

 detennined as such by its connection with the mdimentary 

 pelvis. Strong subveilebral chevi-on bones are placed 

 beneath the interarticular cartilages of the caudal vertebra. 

 The heads of the ribs articulate with the centra of all 

 the vertebrae. The bodies of the ribs are very thick, 

 rounded, and have a remarkably dense and laminated struc- 

 ture. The naiTOw and elongated sternum is an undivided 

 mass of bone, and is connected by ossified sternal ribs with 

 the anterior three pairs of vertebral ribs. 



In the skull the elongated and subcylindiical form of the 



