146 MORPHOLOGY OF THE ORGANS OF VZRTEB RATES. 



to the region near the backbone.' In some forms (^Megaloba- 

 trackiis and some stegocephalans) the ribs are not confined to 

 the trunk region, but from three to eight pairs may occur in 

 the tail. It is to be noted that the pelvis does not articulate 

 directly with the transverse process of the sacral vertebra, but 

 that connection is effected b\- the intenention of a sacral rib, 

 distinct in many forms. In the csecilians ribs occur on ever}- 

 vertebra except the first and last. 



In the amniotes the ribs in the trunk region acquire a much 

 greater development, and, like the hoops of a barrel, extend 



Fig. 154. Pelvis and sacrum of alligator. /, ilium; R, sacral ribs; 

 5', 5", sacral vertebrae. 



around the body cavity. The\" may be ossified throughout their 

 extent, in which case each rib is usual!}' divided into several 

 segments (crocodile ),'- but usually a considerable portion re- 

 mains cartilaginous. \'entrally they may terminate freely, or 

 they may connect with a sternum to be described '.ater. In the 

 great majority of the birds, as well as in some reptiles (croco- 

 dilia, rhynchocephalia), each rib bears a backwardly directed 

 uncinate process, which overlaps the rib behind, thus giving 

 additional strength to the thoracic framework. 



1 Ribs occur in the ventral region of some stegocephali (see p. 14;). and caitilaginoos 

 ventral ribs have been described in Xeciurus and Menop&ma (urodeles). 



- The median segment in crocodiles is not truly ossified, but is cartilage partially 

 calcified. 



