CRETACEOUS LIZARDS. 461 



diameter, although the latter is extended by a short obtuse hypapophysial spine, 

 less developed than in Mosasaunis. Each neurajjophysis presents a large sub- 

 concave facet for articulating with part of the occipital condyle. 



The axis (fig. 3) consists of a long body, including the proper centrum of the 

 atlas, c a, coalesced with that of the axis, c x. The latter develops a hypapophysis, 

 y, to which is articulated a short haemapophysis, //. A compressed vertical ridge- 

 like process (par-diapophysis, d) extends from each side of the centrum ; it may be 

 for the support of a rudimental cervical rib. 



A few of the succeeding vertebrae are characterised by both diapophysis (fig. 4, d) 

 and hypapophysis, y — the latter with a rough articular surface for ligamentous 

 attachment of a haemapophysis, h. 



The diapophysis of the third cervical supports a rib ; and a similar costigerous 

 process is present in the dorsal vertebrae. This series may be conveniently, though 

 artificially, defined by the suppression of the hypapophysis. The zygapophyses 

 (figs. 5 and 6, z z) disappear in the posterior dorsals as in fig, 7. The diminution 

 in vertical and increase in longitudinal extent, together with a descent in position 

 from the side of the centrum, reduce the transverse process to a parapophysis, 

 jo, fig. 8, which characterises the lumbar vertebrae. 



The centrum here becomes triangular, in transverse section, with the base down- 

 ward. There is no 'sacrum' by ankylosis; it is represented by a single vertebra 

 supporting a pair of small rib-hke ilia (fig. 1, h). The pubis, u, is slender, nearly 

 straight ; the ischium, m, is broader, with a short hind process, offering a syndesmosal 

 surface for the ilium and the femur; it also unites with its fellow, completing the 

 inverted ' pelvic arch ' below. The following table gives the kinds and numbers of 

 the vertebrae. 



Number. 

 Atlas and axis 2 



Vertebrae (type 4) with hseraopopliysis, hypapophysis, diapophysis, and zygapophysis 5 



„ (type 5) with hypapophysis, zygapophysis, and diapophysis 7 



,, (type 6) with zygapophysis and diapophysis 18 



„ (type 7) with diapophysis 22 



,, (type 8) with parapophysis 15 



,, (type 9) with parapophysis and unankylosed hasmal arch 24 



,, (type 10) with unankylosed hEemal arch 14 



„ (type 11) with ankylosed hsenial arch 44 



„ (type 12) with centrum and neurapophyses, the latter rudimental or none ... 12 



163 



The vertebrfB of Leiodon are devoid of the accessory zygantral and zygosphenal 

 articulations. In the {eyv Mosasauroids which show them they adhere to the 

 iguanoid type (Ophidia, PI. 2, figs. 34, 35). 



