160 BRITISH FOSSIL REPTILES. 



symphysis tempts to a view of the present evidence of Pliosauriis troclianterius 

 as testifying to something more than specific distinction from the Pliosaurus 

 grandis. 



This species retains more similarity with the type Sauropterygians {PL 

 doUclwdeirus, e.g.) in the proportions of the symphysis and of the number of 

 symphysially located teeth. Nevertheless, modifications in these particulars are 

 presented, though in a minor degree, by species of true Plesiosauri [compare PI. 

 3, fig. 2 {Plesiosaiirus dolichoderius) , with PI. 16, fig. 2 {Plesiosaurus Eawldnsii)']. 



A specimen subsequently submitted to me, includes the part of the maxillary 

 bones, with eight or nine pairs of alveoli at or very near to the hind end of the 

 series, of a smaller individual of the Pliosaurus troclianterius than that to which 

 the lower jaw, PI. 19, fig. 4, belongs. 



This fragment measures 11 inches in length and 6 inches in greatest breadth. 

 It is from the same locality and formation as the larger skull, viz. the Kimmeridge 

 Clay of Kimmeridge. Both specimens have been liberally presented to the British 

 Museum by the discoverer, J. C. Mausel-Pleydell, Esq., F.Gr.S. 



Teeth. — Of the teeth showing the generic modification above defined (p. 152) 

 the best specimen is the subject of Plate 33. It was found in a deposit of 

 Kimmeridge Clay, near Oxford, where fragmentary specimens had been obtained 

 similarly testifying to the bulk and power of some old tyrant of the later 

 Oolitic seas. In one of these specimens, the subject of PI. 19, fig. 7, the 

 circumference of the base of the crown measures 7 inches 6 lines, equalling 

 that of a full-sized tooth of a cachalot- whale {Phijseter macrocephatus) . Of 

 the enamelled crown three inches are preserved and about as much of the 

 cement-covered base, the longest diameter of which is 2 inches 2 lines ; that 

 of the fractured end of the crown is 1 inch 3 lines. The length of an entire 

 tooth is shown in Plate 33 ; the fractured part of the base exposes a pulp-cavity 

 (fig. 2, c) of about 2 inches diameter, with a hard dentinal wall of from 4 to 6 lines 

 in thickness, which gradually decreases to the broken or implanted basal ends of 

 the tooth. The fractured part of the ci-own (fig. 2) exposes a solid and compact 

 mass of dentine. The generic characters of the tooth are boldly pronounced. 

 The smooth facet, defined by strong marginal ridges (fig. 3), shows a few narrower 

 ones, encroaching upon the basal part, where the enamel gives way to the coat of 

 cement covering the long and strong root of the tooth. This is about thrice the 

 length of the crown, and testifies to the strength of implantation. The two sides 

 of the crown (figs. 1 and 2) are continued into each other by an uninterrupted 

 curve ; the ridged enamel angles between these facets and the outer smoother convex 

 side of the tooth are well defined. The terms "convex " and " concave" refer to 

 the longitudinal direction of the tooth ; all the sides are convex across, the outer 

 one being the least so. The unridged surface of the enamel is finely wrinkled by 



