308 THE MECHANICAL FUNCTIONS. 



nal, which, indeed, it scarcely exceeds in its diameter. The 

 bones of the face, are, at the same time, expanded laterally, 

 so as to bear no proportion to the cranial cavity. The head 

 plays on the vertebral column by two lateral articular sur- 

 faces, formed upon the root of each leaf of the occipital bone, 

 while its body, or basilar portion, is scarcely connected with 

 the first cervical vertebra, and has no articular surface. 



In place of ribs, we find only small, slender, detached 

 bones, or rather cartilages, affixed to the extremities of the 

 transverse processes of some of the vertebras. They may 

 be regarded as rudi mental ribs.* 



The pelvis consists of two slender and elongated iliac bones, 

 which are extended backwards, and which, at their anterior 

 extremities, merely touch the points of the transverse pro- 

 cesses of the last vertebra of the back. This vertebra is much 

 broader than the rest, and, although it consists but of a single 

 vertebra, must be considered as a sacrum. The two pubic and 

 ischiatic bones are exceedingly small, but still contribute to 

 form the acetabulum, or cavity for the reception of the thigh 

 bone, at the hinder extremity of the slender bones above 

 mentioned. This is the simplest possible form to which the 

 pelvis can be reduced, while it preserves its attachments to 

 the spine. It presents, in this respect, a more advanced 

 stase of development than that of fishes. 



The connexion of the bones of the anterior extremities 

 with the spine is analogous to that which takes place in rays 

 and sharks: there being an osseous belt formed by the sca- 

 pula, clavicle, and coracoid bone, with the latter of which 

 the humerus is connected. The sternum is large and con- 

 siderably developed; making some slight approach to the 

 expansion it receives in the Chelonia. The radius and ulna 

 are united into oneiDone: the bones of the arm and leg, in 



* The plan of reproduction in these animals requires that the ovary, or or- 

 ffan which contains the eggs, should be capable of enormous dilatation, in 

 order to contain the immense bulk to which these eggs are expanded, pre- 

 viously to their being brought fortli. It was probably in order to make room 

 for this dilated ovary that the ribs have not been developed. 



