BETWEEN TYPICAL EEPTILE8 AND OTHEB ANIMAL3. 169 



developed into a strong erect process, of which there is no trace ia 

 the Crocodile, 



It is probably due to the vertical position of the maxillary bone 

 that the teeth of Lizards are not in sockets, the inner alveolar 

 border being drawn away from them in the elevation of the 

 bone from a horizontal to a vertical position. The teeth of 

 Crocodiles differ but little from front to back ; but in the Draco 

 volans there are kinds which might represent incisors, canines, 

 and molars ; and in many Lizards the premaxillary teeth are 

 sharper, or of different form from the others, and the hinder 

 maxillary teeth undergo a change in the form of the crown 

 quite analogous to what is seen in mammals. 



The fewer neck-vertebrse of Lizards are not usually furnished 

 with ribs ; and when, as in the Skink, ribs are attached to all 

 the vertebrae except the first two, they have only one articular 

 head. The centrum never has the cylindrical form seen in the 

 Crocodile ; and the dorsal vertebrae never have transverse pro- 

 cesses, except in the first few vertebrae of the Dragon, which 

 give ofi" the first ribs to the parachute, where in form they are very 

 unlike those of the Crocodile. The dorsal vertebrae rarely have 

 the vertical, flat, oblong neural spines of the Crocodile ; the 

 neural spines are suppressed in the Dragon, small in the Skink, 

 compressed in front, and oblique in Iguana. In the Monitor, 

 however, the neural spine is very like the Crocodile's throughout 

 the vertebral column. The cup-aud-ball articular vertebral sur- 

 faces are usually transversely depressed and oblique, which is not 

 the case with the Crocodile's. 



Between the dorsal vertebrae which are united with the ster- 

 num, and the neck, are the ribs (with massive ovate heads slightly 

 concave at the articulation) which assist in supporting the shoulder- 

 girdle. The dorsal ribs never include more than three ossified 

 parts, though in Iguana a short unossified cartilage intervenes 

 between the middle and sternal elements, assimilating the rib to 

 that of a Crocodile. 



The caudal vertebrae of Monitor, though far more numerous, 

 are very similar in form to those of the Crocodile, difteriug chiefly 

 in the centrum having a cup-and-ball articulation and in its obli- 

 quity. In Skinks the neural spine is suppressed ; and in Dragons 

 the vertebra is elongated, and its processes scarcely noticeable. 



The sacrum similarly consists of two vertebrae. 



The pectoral arch includes, besides the elements met with in 



