XXIII] CROCODILIA 599 



exceedingly long in comparison with the rest of the- skull, so that 

 the gape is very wide. 



The osteoderms form rings on the tail, but on the body, as in 

 Chelonia, they form a dorsal and a ventral shield separated by inter- 

 vening softer skin. In many Crocodiles the ventral shield is very 

 rudimentary. 



In the general arrangement of the bones and the temporal 

 fossae the skull resembles that of Sphenodon : but there are great 

 differences in the jaws and palate. The maxilla is very long and is 

 armed with conical teeth which are implanted in distinct sockets 

 or alveoli, the bone having grown up round their bases. 



The two palatal folds have met so as to completely divide the 

 upper air passage from the lower food passage : both the palatines 

 and the pterygoids being completely united in the middle line 

 (Fig. 297). The choanae or posterior nares are therefore situated 

 very far back directly over the glottis, whilst the external nostril is 

 at the tip of the snout. 



In consequence of this position of the external nostril the 

 crocodile can lie for hours hidden under the water with only the 

 tip of the snout exposed, and so surprise any unwary animal coming 

 to the water to drink. 



All the cervical and trunk vertebrae and some even of the 

 caudal vertebrae bear ribs. The manner in which these ribs are 

 articulated to the atlas and axis vertebrae throws much light on 

 the relation of these peculiar vertebrae to the rest. Thus we 

 observe that the first pair of ribs are articulated with their heads 

 to the lower part of the atlas, showing that this represents a basi- 

 ventral homologous with the intervertebral cartilaginous pads of the 

 rest of the column. The heads of the second pair of ribs are united 

 to an intervertebral cartilage separating the odontoid process from 

 the centrum of the second vertebra. This cartilage is therefore the 

 second basiventral, and the odontoid process is the first interventral, 

 homologous with the centra of all succeeding vertebrae. The 

 tubercle of each of the second pair of ribs has also an attachment 

 to the odontoid process lying obliquely above and behind the 

 capitular attachment and hence the centrum of the axis vertebra 

 has no rib attached to it. The third pair of ribs have shifted their 

 capitular attachment back on the centrum of the third vertebra 

 (Fig. 298). This backward shifting of the capitular attachment has 

 taken place in all succeeding vertebrae, and the head of the rib is 

 attached directly under its tubercle. In the trunk, as we proceed 



