PLEIOSAURUS. 283 



induced me to found a sub-genus for its reception under the name of 

 Pleiosaurus. 



In the collection of Prof. Buckland, at Oxford, a considerable 

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

 Pleiosaurus brachydeirus is preserved from the Kimmeridge clay for- 

 mation at Market Raisin. The teeth are arranged in separate sockets, 

 in a close and regular series, along the alveolar borders of the inter- 

 maxillary, 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, counting backwards, and the jaw is slightly compressed at this 

 interspace ; the four anterior teeth, thus marked off, occupy the 

 slightly expanded anterior extremity of the upper jaw, but do not 

 present the disproportionately large size which characterizes the 

 anterior teeth in the true Plesiosaurs. After the fifth tooth the 

 sockets progressively increase in size to the twelfth tooth, and, from 

 the fourteenth they begin gradually to diminish in size ; becoming, be- 

 yond 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 developed. 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 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 j 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 extremity of the dental series is complete, but 

 not the anterior one ; thirty-five teeth are present in each pre- 

 mandibular bone. The first, from its large size, I conclude to 

 have been received into the slight concavity at the side of the 



