ON BRITISH FOSSIL REPTILES. 61 



neck are so modified, that the peculiarly elongated proportion of this part of the 

 spine, which characterizes the typical Plesiosaurs, is exchanged for one that 

 much more nearly approaches the opposite condition of the cervical region 

 in the Ichthyosaurs. This abrogation of the main characteristic of the Ple- 

 siosaurs, combined with the more crocodilian proportions of the teeth, esta- 

 blishes the claims of the Pliosaurus to generic distinction. 



In the collection of Professor Biickland, at Oxford, is preserved a consider- 

 able proportion of both the upper and lower jaws of a gigantic specimen of 

 the Pliosaurus, from the Kimmeridge clay formation at Market-Raisin. The 

 teeth are arranged in separate sockets, in a close and regular series, along the 

 alveolar borders of the intermaxillary, maxillary and premandibular bones. 

 Twenty-six sockets may be counted on the most perfect side of the upper jaw; 

 but the series is evidently incomplete posteriorly. An interspace, not quite 

 equal to the breadth of a socket, divides the fourth from the fifth tooth, count- 

 ing backwards, and the jaw is slightly compressed at this interspace ; the four 

 anterior teeth, thus marked off, occupy the slightly expanded anterior extre- 

 mity of the upper jaw, but do not present the excessive size of the correspond- 

 ing teeth in the Plesiosaur. After the fifth tooth the sockets progressively 

 increase in size to the twelfth tooth, and from the fourteenth they begin gra- 

 dually to diminish in size, becoming, beyond the twentieth tooth, smaller than 

 those at the fore part of the jaw. 



The alveolar septa are narrow, and are thinned off to an edge, which is lower 

 than either the outer or inner walls of the sockets : these walls are equally de- 

 veloped. A line drawn transversely across any of the twelve anterior sockets 

 would be transverse to the jaws, but in the remaining sockets it would incline 

 obliquely from without, inwards and backwards. The transverse diameter of 

 the thirteenth socket is one inch six lines ; its antero-posterior diameter is one 

 inch eight lines. 



The extent of the alveolar series in both jaws is nearly three feet; the 

 breadth of the palate at the twenty-sixth tooth is nearly one foot ; the breadth 

 of the upper jaw at the third tooth is four inches and three lines ; the breadth 

 of the socket of that tooth is one inch three lines. 



In the lower jaw of the specimen in the Oxford Museum, the posterior ex- 

 tremity of the dental series is complete, but not the anterior one ; thirty-five 

 teeth are present in each premandibular bone. The first, from its large size, 

 I conclude to have been received into the slight concavity at the side of the 

 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 corre- 

 spond with those of the upper jaw. 



The teeth which are preserved in this magnificent cranial fragment, present 

 the characters above defined. The insei-ted fangs of most of these teeth are 

 four inches in length ; the entire tooth being thus seven inches in length. 

 The ridges which divide the outer from the inner surfaces of the tooth sub- 

 side at the base of the crown ; the fang is smooth ; it assumes a subcircular 

 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 elongated 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 



