ANATOMY OF VERTEBRATES. 53 



thirds of the thoracic-abdominal cavity. They decrease in length 

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

 to short straight pieces, as in the neck, but are not terminally 

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

 The ha3mapophyses in the abdominal region, are subdivided, 

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

 1 plastron ' of transversely extended, slightly bent, median and 

 lateral, overlapping bony bars, occupying the subabdominal space 

 between the scapular, 52, and pelvic, 64, arches. In the tail the 

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

 Ichthyosaurus, ununited both above and below. One Sauro- 

 pterygian genus, Tanystropheus, had the centrum, in certain 

 vertebra?, so long and hollow as to simulate a limb-bone. In 

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

 region, as in the Ichthyosaurus. In a third genus (Nothosaurus) 

 two vertebra? are recognised as sacral by their thick, straight, and 

 convergent pleurapophyses, 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 vertebra?, whence its 

 name, Sphcnosaurus. 



§ 21. Vertebral column of Ophidia. — Amongst existing Reptiles, 

 the Serpents ( Ophidia) surpass all others in the vast number of 

 their vertebra?, which, with incomplete ha?mal arches, compose the 

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



In all these vertebra? the autogenous elements, except the 

 pleurapophyses, fig. 46, /;/, coalesce with one another, and the 

 pleurapophyses become anchylosed to the diapophyses in the tail. 

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

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

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

 ' procoelian ; ' that is, they are articulated together by ball-and- 

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

 47 A, where it forms a deep cup with its rim sharply defined ; 

 the cavity looking not directly forward, 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, h, is developed in different proportions 

 from different vertebra?, but throughout the greater part of the 

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

 crotalus, figs. 47, 47 a, h : 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 



