284 PLEIOSAURUS. 



upper jaw where the diastema separates the fourth and fifth teeth ; 

 there are probably, therefore, thirty-eight teeth on each side of the 

 lower jaw ; counting backwards, on this supposition, the teeth begin 

 to diminish in size beyond the fifteenth, and at the posterior extremity 

 of the series the sockets are less than half an inch in diameter ; 

 in their close arrangement and position they correspond with those of 

 the upper jaw. 



The teeth which are preserved in this magnificent cranial frag- 

 ment present the characters described at the commencement of this 

 section ; the outer smooth surface of the crown of a tooth of the 

 lower jaw is represented at PI. 68, fig. 5 ; the inner surface of two 

 of the teeth of the upper jaw is represented at 5' and 5 ' : the inserted 

 fang of each of these teeth is four inches and a half in length, the 

 entire tooth being thus seven inches in length. The ridges which 

 divide the outer from the inner surfaces of the tooth subside at the 

 base of the crown ; the fang is smooth ; it assumes a sub-circular 

 form, gradually expands for about half its length, and then contracts 

 to its termination, but this is always less pointed than in the fully- 

 formed teeth of the true Plesiosaur. In the old teeth with the elon- 

 gated fang, the pulp-cavity remains open, as in the Plesiosaurian 

 teeth ; it presents at the expanded part of the fang, a narrow elliptic 

 transverse section. In a tooth of the present species, six inches and 

 a half in length, from the Kimmeridge clay at Shotover, the diameter 

 of the persistent pulp-cavity was thirteen lines. In this tooth the 

 flattened surface is polished, but marked with minute shallow wrin- 

 kles ; one of the ridged surfaces, which stood at right angles to the 

 preceding, was traversed by eleven well-marked linear ridges, of unequal 

 length, separated by smooth interspaces of about three times the 

 breadth of the ridges ; the third surface which formed an acute angle 

 with the smooth outer surface was traversed by twelve ridges. These 

 ridges on the inner surfaces of the tooth slightly incline towards the 

 rounded angle dividing these surfaces ; they terminate abruptly ; some 

 cease half-way from the apex of the crown ; about ten are continued to 

 within half an inch of the apex, which is smooth ; the two ridges 

 which divide the flat or smooth side from the ridged surfaces 

 of the tooth are alone continued to the sub-acute apex of the tooth. 



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