504 BRITISH FOSSIL REPTILES. 



In. Lines. 

 Neural spine, length from fore part of base . . .71 



„ ,, hind part of base . . .56 



„ fore-and-aft breadth at mid-length . .18 



„ transverse breadth, at mid-length . .10 



„ „ at tuberous end . .25 



A comparison of such of the above admeasurements as have been recorded of trunk- 

 vertebrae shows that the caudal ones become shortened, at least, at the basal part of the 

 tail. As the length of this appendage would depend upon the number of vertebrae, 

 and especially of those reduced nearly to the centrum, which might again gain in length, 

 it would be premature, on present evidence, to hazard an opinion on this dimension in 

 Omosaurus armatus. But the size of the outstanding parts for muscular attachments 

 indicates great power in the tail, which would probably be exercised, as in the largest 

 living Saurians, in delivering deadly strokes on land, as well as in cleaving a rapid course 

 through the watery element. 



The centrum is transversely elliptical, with both upper and under surfaces sloping from 

 before downward and backward from the terminal articular planes, these being vertical. 

 Of them the anterior (PI. 67, fig. 1, a) is flat, with a slight convexity toward the periphery 

 and a shallow transverse groove at the centre; the posterior surface (PI. 68, fig. 1, h) is 

 more decidedly, though but slightly concave ; the deepest part here, being along a central 

 transverse groove, with a slight upward bend, like that on the opposite surface. A 

 rugged border for the attachment of a capsular ligament projects from two to five lines 

 beyond the articular tract. This, though smoother than any part of the free surface of 

 the centrum, has evidently, by its inequalities or sculpturing, related to a syndesmosal 

 joint, as in the Chelone and Mammalia, not to a synovial one as in Crocodilia. 

 Between the fore and hind borders of the centrum the lower surface is antero-posteriorly 

 concave (PI. 68, fig. 2), the concavity narrowing as it approaches the line of confluence of 

 the pleurapophysis (ib., ih., pi). This line begins below, half way between the under 

 and upper surfaces of the centrum, and extends upward, approaching obliquely the fore 

 surface (ib., a) to overlap and be lost (by anchylosis) in the base of the neurapophysis ; a 

 feeble trace of the primitive separation of this element may be discerned at the hinder 

 outlet of the neural canal (ib., fig. 1, npl). The pleurapophysial line of confluence is more 

 distinctly traceable ; the base of the pleurapophysis, representing the head of the caudal 

 nblet, is broadest below, and there extends nearer the posterior than the anterior surface 

 of the centrum ; but, as it rises, it narrows and leaves a larger pi-oportion of the post- 

 lateral surface of the centrum free. The ' tubercle ' (0 of the rib is a well-marked rough 

 prominence at which the upper border of the rib descends at an open angle with the 

 ' neck ' to its obtuse apex. The under border of the riblet is gently concave lengthwise. 

 No diapophysis has been developed, in this vertebra, to afford abutment to the tubercle. 



