VERTEBRAL COLUMN 439 



pair of chevron-bones, and these are continuous with the inter- 

 central rings of cartilage. 



The atlas and the epistroplinis or axis are of supreme interest. 

 Crocodiles are, in fact, the only animals in which these two 

 vertebrae retain all their constituent hard parts in an almost un- 

 disturbed primitive condition (Fig. 103, 1-4). The basal piece of 

 tlie atlas-ring, the first basiventral or intercentrum, carries a ])air 

 of long ribs attached by their capitular portions. A small knob 

 near the dorsal edge of the rib occurs in many specimens, and 

 is the last remnant of the tubercular portion. The latter was 

 still complete in Jurassic Crocodiles, for instance in Jiretrio- 

 rhynchus (Fig. 1 ?> , 2, t{). The first centrum joins that of the second 

 vertebra as its so-called odontoid process, not directly, however, 

 but by the intercalation of the complete second basiventral, repre- 

 sented by a cartilaginous disc, and by a large unpaired pyramidal 

 piece (Fig. 103, 3,-). This, serially homologous with the ventral 

 half of the atlas -ring, is the second basiventral intercentrum, 

 wedged in from below between the odontoid process and the 

 second centrum, with which it soon fuses. Moreover, it carries 

 the capitulum of the second rib (2, C^r), the tuberculum of which 

 is articulated with a facet of the second neural arch in Jurassic 

 Eusuchia (^2)- In recent Crocodiles this tubercular portion is much 

 reduced, and, curiously enough, is attached to a knob which 

 belongs to the odontoid piece or first centrum. This shifting 

 explains the apparently anomalous condition that " the atlas of 

 the Crocodiles carries two pairs of ribs, the second vertebra none." 

 To complete the account of the atlas we have to mention the 

 separate unpaired piece whicli lies upon the two neural arches. 

 It is the detached neural spine, and not the remnant of a 

 " pro-atlas." 



The first and second ribs {R-^ and /.'.,), at least in the recent 

 forms, are very long and are quite movable. Tiiose of the next 

 five cervical vertebrae are firmly fixed, short, and adze-shaped. 

 The eighth and ninth are again long, and make the transition to 

 the thoracic ribs, which are mostly eight in number, some with 

 uncinate processes. Then follow several shorter or floating ribs, 

 mostly two or three pairs. The next following three presacral 

 vertebrae carry no ribs. The two sacral and the caudal ribs 

 have already been mentioned. 



As a rule the vertebral column of recent Crocodiles, Alligators, 



