ANATOMY OF VERTEBRATES. 53 



tliii-ils of tlic thoracic-abdominal cavity. They decrease in lengtli 

 and curvature as they approach the tail, where they are reduced 

 to short straight pieces, as in the neck, l)ut are not terminally 

 expanded ; they cease to he developed near the end of the tail. 

 The hromapophyses in the abdominal region, are subdivided, 

 and with the hajmal spine or median piece, form a kind of 

 'plastron' of transversely extended, sliglitly bent, median and 

 lateral, overlapping bony bars, occupying the suljabdominal space 

 l)etween the scaprdar, 52, and pelvic, 64, arches. In the tail the 

 haimapophyses are short and straight, and remain, as in the 

 Iclithyosaurus, ununited l)oth above and below. One Sauro- 

 pterygian genus, Tiimjstrophai.s, had the centrum, in certain 

 "S'ertebrre, so long and hollow as to simulate a limb-bone. In 

 another genus, (^PUosaurus) they were as short, in the cervical 

 region, as in tlio Iclttlnjo.iaurus. In a third genus (NotJwsaurus) 

 two vertel)rffi are recognised as sacral hj their tliick, straight, and 

 convergent pleurapophyscs, of which the first overlaps the second. 

 In a fourth genus the wedge-shaped hypapophyses occur at the 

 lower interspaces of the dorsal and lumbar vertebrfc, whence its 

 name, Sjj/ienosaurus. 



§ 21. Vertebral column of Ojjhidia. — Amongst existing Keptiles, 

 the Serpents ( Oj/Jridict) surpass all others in the vast number of 

 their vertebra;, which, with incomplete hajmal arches, compose the 

 skeleton of the long, slender, limbless trunk, fig. 46. 



In all these vertebra; the autogenous elements, except the 

 pleurapo])hyses, fig. 46, pi, coalesce with one another, and the 

 pleurapophyscs become ancliylosed to the diapophyses in the tail. 

 There is no trace of suture between the neural arch, fig. 47, 

 7is, z, and centrum, c. The outer substance of the vertebra is 

 compact, with a smooth or polished surface. The vertebra; are 

 ' procGjlian ; ' that is, they are articulated together l)y ball-and- 

 socket joints, the socket being on the fore jiart of the centrum, fig. 

 47 A, where it forms a deep cup with its run sharpl}^ defined ; 

 the cavity lof)king not directly forAvard, but a little downward, 

 from the greater prominence of the upper border : the well-turned 

 prominent ball terminates the back part of the centrum rather 

 more obliquely, its aspect being backward and upward, fig. 47, 

 c. The hypapophysis, A, is developed in different proportions 

 from different ^'ertcbraj, but throughout the greater part of the 

 trunk presents a considerable size in the cobra, 46, liy, and 

 crotalus, figs. 47, 47 A, It: it is shorter in the python and boa. 

 A vascular canal perforates the under surface of the centrum, and 

 there are sometimes two or even three smaller foramina. In the 



