THE REPTILIAN CLASS OF THE MOSASATTRLD^E. 705 



The anterior hypapophysis of the atlas forms the lower third of 

 the cup for the occipital condyle, the two upper thirds being modi- 

 fied praezygapophyses of the neural arch. A short thick diapophysis 

 projects behind the praezygapophysis, and a small tubercle from the 

 outside of the base of the postzygapophysis. The neurapophyses 

 are confluent by their apices, without developing a spine. The 

 centrum of the atlas (" odontoid process ") sends downward and 

 backward a longish compressed posterior hypapophysis ; the anterior 

 one has more fore-and-aft extent, and is in the form of a vertical 

 plate or keel. 



The axis and four following vertebrae have each a hypapophysis 

 not confluent with the centrum, but articulated to a projection from 

 the hinder border, so as to seem wedged into the interspace between 

 its own and the next vertebra. 



The fifth vertebra, counting from the occiput, has free pleura- 

 pophyses ; and these elements are present and free in the succeeding 

 vertebrae, to the sacrum. They then suddenly gain thickness, with 

 more length in the first sacral, and are confluent with the diapo- 

 physes. The ilium is a similarly thickened homotype, or " serial 

 homologue," of the second portion of the fully developed thoracic 

 costal arch ; and the pubis is a similarly modified homotype of the 

 haem apophysis, sternal rib, or costal cartilage of the type thoracic 

 segments. 



The completion of the arch by the latter element occurs in the 

 ninth vertebra, and ceases at the sixteenth, which is followed by eight 

 vertebrae with pleurapophyses only. The transverse process of the 

 first and ten following caudal vertebrae is as long as the di-pleur- 

 apophysis of the last trunk- vertebra, and includes the same elements 

 coalesced. The diapophyses disappear at the twenty-first caudal 

 vertebra. The haemal arch shows the usual type of " chevron bone," 

 with a spine equalling the neural one above. 



The haemapophyses articulate to tubercles at the back part of their 

 centrum, and leave no articular mark on the fore part of the following 

 centrum ; but their base extends over the intervertebral space. 



The main vertebral character by which the existing Sea-lizards 

 (Amblyrhynchns) differ from the terrestrial or arboreal Iguanians * 

 is in the absence of zygosphene and zygantrum, and the limitation of 

 the articular processes of the neural arch to the normal prae- and 

 post-zygapophyses ; according to the rule of the Lacertian order. 



And here we gain another capital test of the affinities of the Mosa- 

 saurians. In the posthumous edition of Cuvier's ' Legons d'Anat. 

 Comparee,' the editors added the fact that in the vertebrae of Ophi- 

 dians the articular processes were double, the superadded pairs 

 being situated internal to the normal ones, and at the base of the 

 spinous process f . 



* Anat. of Vertebrates, vol. i. p. 57, fig. 48, "trunk-vertebra of Iguana tuber- 

 culata. 



t " Les apophyses articulaires sont doubles ; les unes exterieures, represen- 

 tant les apophyses articulaires ordinaires a facettes horizontals ; les secondes 

 inte>ieures sont situ£es a la base de l'apophyse epineuse." Vol. i. 8vo, 1835, 

 p. 216. 



