KIMMERIDGIAN PLIOSAURS. 161 



short wavy risings, frequently joining or reticulate, rather affecting the longitudinal 

 course. The same character is presented by the enamel covering the contiguous 

 parts of the other sides of the tooth, and extends furthest in that represented 

 in fig. 1, Plate 33. The enamel, which is a mere film at the base of the crown, 

 gains thickness towards the apex; its adhesion to the dentine is helped by 

 numerous fine, wavy, longitudinal, sub-equidistant, linear risings on the surface of 

 that substance. The slight outer convexity is uniform ; the inner curve is wavy, 

 passing from the slight concavity at the crown to a slight convexity at the junction 

 of crown and fang, then again becoming slightly concave. 



The subject of Plate 33, now in the British Museum of Natural History, 

 was presented to the Trustees by the Hon. Robert Marsham, F.G.S. This tooth 

 exemplifies its complete state of formation, the entire fang has been develojDed, 

 and the unworn crown shows that the time had not arrived for the absorption of 

 the root through pressure of a successional tooth, which undermining process is 

 usually concomitant with the loss of efficiency of the dental instrument through 

 the wear of the crown. It accordingly presents a total length of one foot, a third 

 of which is formed by the crown, the rest by the root. This cement-covered part 

 expands for the coronal half of its extent to a diameter of 3 inches, which is the 

 thickest part of the tooth ; it then gradually contracts to the thin borders of the 

 base of the pulp-cavity, where probably an additional inch of the tooth has been 

 broken away. 



In excuse for the foregoing details, which may be deemed tedious, I plead the 

 rarity of a specimen so complete as to yield the collective dental characters, and 

 the frequency of fragments, of which any one showing part of the enamelled surface 

 of the tooth may now have the nature of its markings and its position in the 

 tooth-crown recognised ; thus it may throw light on the mesozoic bed, which is 

 characterised by the present Sauropterygian genus. 



Finally, I may refer to the evidence of provision, in Pliosaurus, for a worn or 

 broken lethal weapon, by a successional tooth ; and this provision, common to 

 Reptilia, relates to the lengthened period during which the individual depends 

 upon its teeth for sustenance. 



In size, in general shape, in vmiformity of character throughout the mandi- 

 bular series of teeth, the existing Cachalots most resemble the Pliosaurs. Those 

 whales are also deemed to be naturally long lived, but their term of life must 

 depend on their enjoyment of the organs by which they get their food, and there 

 is no evidence of the first fully-developed series of teeth ever being replaced by a 

 second set. The cachalots were not favoured with the mode of supply for lengthened 

 individual life which is exemplified in modern as in the ancient aquatic Saurians. 



For how many years the individual Pliosaur dominated its latitudes and preyed 

 upon marine contemporaries we may not Ije able to determine, but its dental 



1^ A. 



