320 THE VERTEBRATE SKELETON. 



a small bone which underlies the articular and supra-angular 

 on the inner side of the jaw. The dentary (fig. 59, 15) forms 

 the anterior half of each ramus, and is the largest bone of the 

 mandible ; it is fused with its fellow at the symphysis in front, 

 and extends back below the supra-angular. The splenial is a 

 small bone lying along the middle half of the inner side of 

 each ramus of the mandible. 



(4) THE HYOID. 



With the hyoid apparatus is included the eolumella. 

 This forms a minute rod of bone, one end of which is ex- 

 panded and fits into the fenestra ovalis, while the other end,, 

 terminated by a triradiate piece of cartilage, is attached 

 to the tympanic membrane. The structure is as a whole 

 homologous with the auditory ossicles of mammals and the 

 hyomandibular of fish. 



The hyoid consists of a median unpaired portion, formed 

 of two pieces of bone, the basi-hyal (fig. 59, C, 16) in front, 

 and the uro-hyal (fig. 59, C, 17) behind, the two being placed 

 end to end and terminated anteriorly by an unpaired carti- 

 laginous plate, the os entoglossum. At the posterior end 

 there come off" a pair of long posterior cornua, each of 

 which consists of two pieces, a longer basibranchial (fig. 59, 

 C, 18), and a shorter ceratobranchial. For the homology of 

 these parts see p. 336. 



THE RIBS AND STERNUM. 



The last two cervical vertebrae bear long movable ribs 

 which articulate by distinct capitular and tubercular processes, 

 but do not meet the sternum. The thoracic ribs are eight in 

 number, and each is divisible into a vertebral and a sternal 

 portion. The first five thoracic ribs are flattened curved bars 

 of bone, which articulate by a prominent capitulum with 

 the centrum of the corresponding vertebra, and by a tuber- 

 eulum with the transverse process. Projecting backwards 

 from each is a large hooked uncinate process. The last three 



