4?12 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 20, 



If we suppose some sudden diffusion of mephitic gas through the 

 waters, produced by submarine volcanic action, this would be a 

 cause sutficient for the immediate destruction of all animals within 

 its influence, and during its continuance would also prevent other 

 animals entering within its poisonous precincts, thus allowing time 

 for the dead bodies to be quietly deposited in the manner before 

 suggested. 



To these or some such causes and effects we are indebted for the 

 means of examining the remains of the Enaliosaurians in the very 

 perfect condition in which they are generally found. 



In the descriptive detail which the admirable state of preserva- 

 tion of the specimen before us enables me to give, I shall adopt the 

 nomenclature of Professor Owen *, as being most applicable to this 

 particular order. 



Plesiosaurus megacephalus. 



Animal with spinal column consisting of ninety-four vertebras in 

 the following divisions: — cervical twenty-nine, dorsal and lumbar? 

 thirty-four, sacral and caudal thirty-one. Total ninety-four. 



Head large, in length equal to two-thirds that of the neck, and 

 one-sixth of the whole length of the body. 



The whole length of the animal from the anterior portion of the 

 muzzle to the posterior extremity of the tail, measured along its 

 curvatures, is sixteen feet three inches. 



The superior portion of the head, posterior to the orbits, is so 

 much crushed as to prevent any examination. 



The teeth, of which more than sixty can be counted in the two 

 jaws anterior to the line of the orbits, are finely striated towards 

 their apex in the young state, but perfectly smooth in the matured 

 teeth. 



The sixth tooth from the front in the lower jatv outside the socket 

 is in length 1 inch y^ths, and the diameter at the base ^ths inch. 



In this specimen there is an inner row of small mucronate teeth 

 situated parallel to the principal or external teeth f. 



The columellar bone is in this specimen clearly displayed, and is 

 separated at its superior extremity, and slightly shifted at its inferior, 

 now lying on the same plane as the whole inferior surface. 



* Vide Report on the ' British Fossil Reptiles,' laid before the British Associa- 

 tion in 1839. 



t Professor Owen regards these as the young teeth in progress of growth, and 

 describes them as follows : — " The germs of the successional teeth are developed 

 at the inner side of the bases of the old teeth, but do not penetrate these teeth ; 

 the apices of the new teeth make their appearance through foramina situated at 

 the inner side, and generally at the interspace of the sockets of the old teeth. 

 Here, therefore, the growing teeth may be included in closed recesses of the os- 

 seous substance of the jaw, and emerge through tracts distinct from the sockets 

 of their predecessors." — Odontography, p. 285. 



And again, — " The dentition of the Plesiosaurus differs from that of the Croco- 

 dile, inasmuch as the new tooth, instead of emerging from the pulp-cavity of the 

 old tooth, or even from the same socket, protrudes its apex through a distinct 

 foramen at the inner side of the alveolus of its predecessor." — Ibid. p. 282. 



