218 



EEPTILIA 



CLASS III 



Fig. 320. 



Crocodüus vulgaris, Cav. Receiit ; Egypt. Anterior 

 cervical vertebrae, I-IV, n, Lateral piece of atlas ; pod, 

 Odoiitoid process (=centrum of atlas); 'pr, Proatlas ; ri-H, 

 Cervical ribs ; x. Basal piece of atlas ( = intercentrum). 



basiocciput and represents a rudimentary proatlas. The basal piece Supports 

 a pair of styliform, single-headed ribs, directed downward and backward. 



Properly speaking the atlantal 

 centrum is represented by the 

 odontoid process of the axis. The 

 seven succeeding cervicals bear 

 strong neural spines, transverse 

 processes, zygapophyses, parapo- 

 physes, and frequently also ventral 

 keels or true hypapophyses. The 

 ribs borne by these vertebrae are 

 all short, hatchet - shaped, and 

 double-headed. In the first two 

 dorsal vertebrae the parapophyses 

 and transverse processes are separ- 

 ated by a narrow interspace, and 

 the latter become more elongated, 

 passing posteriorly (caudal). The 

 parapophyses gradually ascend, 

 passing on to the neural arch in the third dorsal vertebra, and thereafter 

 both articular facettes for the double-headed ribs are borne by the transverse 

 processes. These facettes continue to approximate, and finally become 

 merged with each other in the posterior dorsal vertebrae, so that the last 

 rib is single-headed. The lumbars, of which there are from four to six, 

 have well-developed transverse processes, but are non-costiferous. The short 

 and stout sacral ribs are as a rule distally expanded, and suturally united to 

 their centra and neural arches. The vertebrae of the anterior half of the tail 

 bear moderately developed transverse processes, attached like the sacral ribs to 

 the neuro-central sutures. All except the anterior and a few of the posterior 

 caudals bear chevron bones. 



The first and second pairs of dorsal ribs are attached to the sternum, which 

 is a large, rhomboidal, cartilaginous plate lying immediately below and 

 behind the interclavicle (episternum). The succeeding five to seven pairs 

 are connected by means of sternal ribs with the so-called abdominal sternum. 

 The sternal ribs are followed by seven pairs of abdominal ribs, of which the 

 posterior is the most strongly developed, and unites with the pubic cartilage. 

 Cartilaginous uncinate processes are borne by the dorsal ribs. 



The external cranial bones are almost always coarsely sculptured. The 

 oval or rounded orbits, which in the preceding sub- Orders are invariably 

 enclosed by bone, are here confluent with the lateral temporal vacuities. The 

 size of the latter is usually in inverse proportion to that of the supratemporal 

 vacuities, and the latter as a rule are smaller than the orbits. The parietal 

 and frontal are unpaired in the adult condition, and there is never a parietal 

 foramen. The squamosal is a triangulär bone forming the postero-external 

 border of the supratemporal vacuity. Postfrontals, prefrontals, and lachrymals 

 are all variable in size. The nasals are long, triangulär bones, usually reaching 

 the premaxillae, and sometimes even the external nostril in front, and applied 

 against the acute median process of the frontal posteriorly. The nostril 

 opening is undivided, terminal, and enclosed by the short, paired premaxillae. 

 The quadrate is broad and immovably united with the surrounding Clements. 



