144 REPORT— 1841. 



■vertebral space 4|- inches, we obtain the length of 12 feet 6 inches for the 

 tail of the Iguanodon. On the foregoing data, therefore, we may liberally 

 assign the following dimensions to the Iguanodon : — Feet. 



Length of head, say 3 



Length of trunk with sacrum 12 



Length of tail 13 



Total length of the Iguanodon ... 28 

 The same observations on the general form and proportions of the animal, 

 and its approximation in this respect to the Mammalia, especially the great 

 extinct Megatherioid or Pachyd.ermal species, apply as well to the Iguanodon 

 as to the Megalosaurus. 



Order LACERTILIA. 



Leaving now the gigantic Saurians constituting the order Dinosauria, 

 above characterized, and establishing in several important points of their os- 

 teological structure the transition from the Crocodilian to theLacertian order, 

 I next proceed to notice the remains of those extinct Reptiles, which manifest 

 in the enduring parts of their organization a closer affinity to the extensive 

 and varied order of the smaller and lower organized Saurians which are dis- 

 tributed over all the warmer parts of the present surface of the earth. 



The ancient representatives of the Lacertian order are for the most part of 

 gigantic size, and deviate, like many of the ancient Crocodilians, from ex- 

 isting Lizards, by very remarkable characters of the vertebrae, teeth, and 

 dermal bones. 



Genus Mosasaurus. 



Commencing with the species which retain the ordinary ball and socket 

 structure of the vertebrae, the gigantic Mosasaurus first claims attention. Two 

 vertebrae which have the anterior articular facet concave,' the posterior con- 

 vex, and the other characters of this genus, are preserved in the Mantellian 

 Collection. They are from the chalk formation in Sussex, and have been re- 

 ferred by Dr. Mantell to the genus Mosasaurus. 



Genus Leiodon. 



Hitherto no teeth corresponding with those of the Mosasaurus Hoffmanni 

 of St. Peter's Mount near Maestricht, have been discovered in the chalk 

 formations of England. The teeth of the Pliosaur have, in some instances, 

 been mistaken for those of the Mosasaur. 



The teeth from the chalk of Norfolk, figured and described in my 'Odonto- 

 graphy*' as representatives of the gen\xs Leiodon, make the nearest approach 

 to the characters of those of the Mosasaurus. They are about one half the 

 size of the maxillary teeth of the Mosasaurus Hoffmanni, and difi"er more es- 

 sentially in having their outer side as convex as the inner side, the transverse 

 section of the crown being elliptic, the pointed extremities of the ellipse cor- 

 responding with two opposite longitudinal trenchant ridges, which separate 

 the outer from the^ inner side of the tooth. The crown expands at the base, 

 which is circular, and is anchylosed to a conical process, developed from the 

 broad alveolar margin of the jaw. In this, which is termed the " acrodoi^t " 

 type of dentition, the Leiodon corresponds with the Mosasaur. It is proba- 

 ble that the vertebrae of the two extinct reptiles may have corresponded in 

 form ; and it is possible that those from the English chalk, hitherto referred 

 to the Mosasaurus, may appertain to the same species as the teeth here de- 



* P. 261, pi. Ixxii. 



