THE REPTILIAN CLASS OF THE MOSASALRID^. 707 



greater expanse of the upper ends of the atlantal neurapophyses, 

 which nnite by bone to form a broad snbquadrate platform above the 

 neural canal *. I have not seen this character of the atlas in any 

 Lacertian. Save in the non-union of the apices of the neurapophyses, 

 Mosasaurus agrees closely in the type of its atlas vertebra with that 

 order of JReptilia. 



Beyond the axis, the vertebrae in most Lacertilia offer well-marked 

 modifications. Those with the diapophyses and with a hypapophysis 

 wedged into the lower part of the interspace between this centrum 

 and that of the next vertebra are, in Amblyrhynchus, two in number ; 

 then comes a vertebra with a pleur apophysis projecting freely from 

 the end of the diapophysis ; and the centrum continues to show the 

 hypapophysial pit upon both the fore and hind part of its under 

 surface. At the fourth vertebra from the axis the hind pit disap- 

 pears, the pleurapophysis elongates ; at the seventh vertebra from 

 the axis the pleurapophysis is connected, through a haemapophysis, 

 with the sternum. If this vertebra be reckoned the first dorsal, 

 then AmhlyrJiynclius has eight cervicals ; if the foremost vertebra 

 with free pleurapophyses is the first dorsal, then this Sea-lizard has 

 but four cervicals. 



There are eight vertebrae which may be called " typical " in having 

 their haemal arch complete ; and in these it is to be noted that the 

 partially ossified portion of the rib is in two parts, or formed by a 

 transversely divided haemapophysis. Then follow the eight vertebrae 

 with progressively shortening pleurapophyses, terminated each by a 

 free cartilaginous portion. Not any of the trunk- vertebrae beyond 

 the sixth, including the atlas and axis, has a hypapophysis. 



In Ophidia the hypapophysis is exogenous. In Python tigris, with 

 291 vertebrae, such inferior process (fig. 24, liy) is present in the 74 

 anterior vertebrae. In Boa constrictor, with 305 vertebrae, a hyp- 

 apophysis is developed in the 60 anterior ones. In Crotalus horridus, 

 with 194 vertebrae, 168 develop hypapophyses as long as the neural 

 spines, and all these vertebrae support movable ribs. In Naia there 

 are as many vertebrae similarly characterized. The presence of the 

 long hypapophysis in the dorsal vertebrae of Laophis crotaloides sup- 

 ported the conjecture that it might belong to the poisonous section 

 of Ophidia f . 



Passing over the two sacral vertebrae of ArnblyrhyncTms, we then 

 come to the caudals, in which the character and position of the 

 haemal arch has already been indicated. 



In Ophidia the tail- vertebrae have not the freely jointed haemal 

 arch with the concomitant pits or articular surfaces. In Python 

 the caudal hypapophyses form a transverse pair confluent with the 

 centrum above, descending parallel to each other, and terminating 

 freely and apart below. The transition from the single hypapophysis 

 of the trunk-vertebrae is made by the bifurcation of the tubercle to 

 which the hypapophysis is reduced in the hinder abdominal vertebrae 



* Monogr. Crocodilia & Ophidic,, Palseontographical vol. for 1850, pi. xiv. 

 figs. 38, 39. 



t Quart. Journ. Geol. Soc. vol. xix. p. 197, pi. iv. fig. 3. 



