Dr. T. Wright on the Paleontology of the Isle of Wight. 233 



which is directed towards the root ; the internal surfaces of the 

 anterior and posterior borders (a) are abruptly truncated, appa- 

 rently by dentrition, and near the base of the posterior border 

 there is an oblique fold or depression, close to which are marks 

 of abrasion by dentrition : the unequal extent of the enamel on 

 the external and internal surfaces of the crown proves that the 

 external plate of the ramus of the jaw was deeper on the external 

 than on the internal side. 



The root (a, b, c, e) is nearly cylindrical ; from the external ter- 

 minal fold of enamel to its fractured part, it measures 1 inch 

 and y°ffths ; its surface is much concealed by the matrix, and has 

 masses of pyrites adhering thereto ; it forms a hollow cylinder 

 (e) which inclosed a pulp-cavity ; the structure and form of the 

 root is that of a tooth which was implanted in a distinct alveolus 

 of a large and powerful jaw. Part of the apex is broken off, the 

 position of which we have indicated by dotted lines ; there can 

 be no doubt that it was sharply pointed, and that this tooth was 

 an instrument destined to pierce the soft structures of other ani- 

 mals, and consequently that it belonged to an extinct genus of 

 carnivorous reptiles. 



Diadema Autissiodorense, Cotteau. 



Syn. Diadema Autissiodorense ', Cotteau, Cat. Method, des Eohi- 

 nides dans l'e'tage Neocomien, p. 5. 



Test pentagonal, depressed; interambulacral tubercles a little 

 larger than those of the ambulacral areas, more especially as 

 they approach the ovarial disc ; interambulacral areas with two 

 rows of primary tubercles, and two incomplete series of 

 secondary tubercles at the ambulacral sides thereof, which 

 gradually disappear on the upper surface ; ambulacral areas 

 prominent, with two rows of primary tubercles much dimi- 

 nished in size at the upper surface. Pores in a double series 

 near the ovarial disc and at the circumference of the mouth. 



Height -njths of an inch ; transverse diameter j^ths of an 

 inch. 



Description. — In its general outline this beautiful Urchin re- 

 sembles D. depressum of the Inferior Oolite, but the details of its 

 structure are very distinct from that form. The circumference 

 is pentagonal from the convexity of the ambulacral areas, and the 

 base and summit are much depressed. 



The interambulacral areas are one-third broader than the 

 ambulacral ; two rows of primary tubercles occupy the centre of 

 the plates ; there are about ten pairs of tubercles in each area, 

 which are of a moderate magnitude, and gradually diminish in 

 size from the circumference to the base and summit ; the mam- 



