THE SIRENIA. 331 



not more than three lumbar vertebras ; 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 processes. 



The zygapophyses of successive vertebrae articulate to- 

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

 gions, the postzygapophyses disappear and the prezygapo- 

 physes are small, and neither overlap, nor embrace, the spine 

 of the antecedent vertebra. The posterior moiety of the spine 

 thus acquires considerable flexibility. There is no true sacrum, 

 the vertebra called " sacral " being only determined as such 

 by its connection with the rudimentary pelvis. Strong sub- 

 vertebral chevron-bones are placed beneath the interarticular 

 cartila2:es 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 structure. The narrow 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 subcylindrical form of the 

 cranial cavity is worthy of notice, as it strongly contrasts with 

 the form of the brain-case in the Cetacea. The supraoccipital 

 is very large and slopes upward and forward a long way on to 

 the upper surface of the skull ; but it does not separate the 

 parietal bones ; which, as usual, unite in the sagittal suture. 

 The frontals are prolonged into broad supraorbital processes. 

 The nasal bones are abortive, and, in the dry skull, the external 

 nares are very wide, and look upward. The tympanic bone is 

 a thick hoop, anchylosed with the periotic bones, and readily 

 comes away from the skull with them. The zygoma is 

 enormously stout. The premaxillj© constitute a large portion 

 of the boundary of the gape ; and the lower jaw has a high 

 ascending portion, with a large coronoid process. 



The scapula has a distinct spine occupying the ordinary 

 position. There are no clavicles. The humerus has its distal 

 end fashioned into articular surfaces, upon which the radius 

 and ulna are freely movable. The pollex is rudimentary, and 

 the other digits have no more than three phalanges each. 



The pelvis is rudimentary, the bones which represent the 

 ossa innominata being connected by their proximal ends with 

 the transverse processes of the last of the precaudal vertebrae. 

 They are disposed vertically to the axis of the body. No 

 trace of the hind-limbs has been observed in any of the exist- 

 ing Slrenia. 



