CRETACEOUS FORMATIONS. 



41 



as is shown by fig. 6 as compared with fig. 4, and by the following admeasurements 

 of one of the caudal vertebrae : — 



Length of the centrum 

 Vertical diameter of the convex end 

 Transverse diameter of ditto . 

 Length of the base of the neural arch 

 Length of the base of the haemal arch 



Inch. 



Lines 



1 



7 



1 



5 



1 



3 



1 











9 



The hsemapophysis (h) swells outwards at its origin, before it bends downwards, 

 backwards, and inwards to unite with its fellow in order to complete the arch. The 

 area or span of this arch has been considerable, as in the vertebra, fig. 5, T. IX, and 

 as it is in the Mosasaitrus Hoffmanni : it is probable that the spinous process continued 

 from it had a corresponding remarkable length, but of this the fractured condition of 

 the specimen affords no proof. The lateral surface of the centrum is smooth, with 

 many small vascular perforations. There is a slight but well-marked rising above the 

 base of the hsemapophysis, at d, which seems to indicate a last rudiment of the 

 diapophysis. A narrow vertical ridge (V) extends about two lines from the border of 

 the posterior convex surface, as if it were indicative of the limits of an epiphysis 

 which had formed that surface. The border of the anterior concave surface has been 

 worn or broken away. A linear impression gives also an indication of an epiphysis in 

 the dorsal vertebra of the Mosasaurus Hoffmanni. The slight degree of concavity and 

 convexity of the terminal articular surfaces of the centrum in these vertebrae is 

 characteristic of the genus. In their special characters, the small vertebrae from 

 Lewes correspond with the vertebrae attributed to the Mosasaurus gracilis, which are 

 longer and more slender than those of the Mosasaurus Hoffmanni. 



Genus. — Leiodon, Owen. 



'Odontography,' p. 261, pi. lxxii, figs. 1 and 2. 

 ' Report on British Fossil Reptiles,' Trans. Brit. Association, 1841, p. 144. 



The teeth from the chalk of Norfolk, surmised by Dr. Mantell, from "their 

 symmetrical, conical form, and other characters," to belong to an unknown reptile, or 

 to a sauroid fish ;* and described and figured in my ' Odontography'! as character- 

 istic of a new genus of Mosasauroid Reptiles, under the name of Leiodon,\ presented 



* Wonders of Geology, ed. 1339, vol. i, p. 339. f v ol. i, p. 261, pi. lxxii, figs. 1 and 2. 



X Aetos, smooth, o&ovs, tooth. 



6 



