CRETACEOUS FORMATIONS. 71 



of its outer surface is shown, which gives it the appearance of forming a crust about a 

 line in thickness, of a different texture from the rest of the bone. 



The fragment, T. XXIII, fig. 3, is a portion of the right premandibular bone, 

 showing a cast of the dental vasculo-nervous canal, and the outlets terminating at the 

 orifices on the outer side of the bone. 



The teeth, supposing them to have been correctly restored, decrease in size, as in 

 the Ichthyosaurus communis, near the anterior end of the dentary piece. 



The largest tooth in this portion of the jaw, placed one foot from the anterior end, 

 has a crown eleven lines in length, straight, conical, rather obtusely pointed, five lines 

 and a half in diameter, with numerous, not very sharp, but close-set ridges, narrowing 

 as they approach the summit, and subsiding before they attain it. The cement- 

 covered base continues to expand as it descends, with a smooth exterior for about one 

 third of its extent from the crown, and with coarse longitudinal striatums or wrinkles 

 over the rest of its extent. The surface of the base in most of the teeth, like the 

 surface of the bone, is coated by a firm crust, sparkling with minute crystals of pyrites. 

 In the attempt to remove this coating, the parts have been more or less injured, so 

 that the precise character of the external markings, and original shape of the thickened 

 fang, cannot be ascertained. The transverse section of the crown of the tooth is 

 circular at its apical half, but widens into a full ellipse towards the base ; that of the 

 smooth beginning of the base is a modified ellipse, which in the rougher and more 

 expanded part of the base, seems to take on a subquadrate form. 



The teeth differ in size at different parts of the jaw ; in the first or foremost of the 

 series the crown of the tooth is only four lines in length in the lower jaw, and it 

 gradually increases to eight and ten lines in length, — the total length of the longest tooth 

 being two inches and a half. Some of the scattered teeth adherent to the present 

 fragments having very short and thick crowns. In fig. 4, the crown is as wide at 

 its base as it is long : a portion of the thick cement has been removed from the fang 

 just below the crown, and exposes the grooved exterior surface of the dentinal base 

 of that part of the tooth. 



In the Ichthyosaurus communis, the teeth of which most resemble those of the present 

 species from the chalk, the crown of the tooth tapers more gradually to the apex ; 

 and the enamel ridges are immediately continued upon broader rounded ridges of the 

 cement-covered fang, which become more strongly marked as the fang recedes from 

 the crown, and are divided by deep grooves, giving a fluted character to the base of 

 the tooth, which is proportionally less expanded, and retains more of the circular form 

 in transverse section. (See T. IV, fig. 17.) 



In the few more or less imperfect teeth of the Ichthyosaurus from the chalk, which 

 I had seen whilst drawing up, in 1838, my Report on 'British Fossil Reptiles,' the 

 differences above specified were not manifested so clearly as in the more numerous 

 and complete examples which have since been submitted to me. The smooth exterior 



