CRETACEOUS FORMATIONS. 43 



pterygoid teeth is more convex than the nearly flat outer side of the maxillary teeth. 

 They resemble, in fact, in their transverse section, the lower maxillary teeth of the 

 Mosasmirus Dixoni. The alveolar border to which the pterygoid teeth are attached 

 in the Mosasmirus Hoffmanni, is moderately convex towards the cavity of the mouth ; 

 the alveolar tract is relatively thicker or broader than on the jaws, and the germs of 

 the new pterygoid teeth appear almost like a second small row on the outer side of 

 that row which is in place, being less close to the teeth they are destined to replace 

 than they are in the maxillary series. 



The teeth in question from the English Chalk, differed in the shape of their 

 crowns from the pterygoid teeth of the Mosasaurus Hoffmanni, and the alveolar 

 border to which they were attached, more resembled that of the dentary piece of 

 the lower jaw. In the smoothness of the enamelled crown, its compressed elliptical 

 form and trenchant borders, which, when magnified, presented a fine serration, the 

 teeth in question, approached to the characters of those of Geosaurus, as much as they 

 deviated from those of Mosasaurus. Both Mosasaurus and Geosaurus afford types of 

 the acrodont mode of dental attachment. Had only the teeth and portions of the 

 jaws of the Geosaurus been known they might have been registered, on such limited 

 evidence, as having belonged to a species of Mosasaurus distinct from the Mosasaurus 

 Hoffmanni, and the Anatomist, Soemmerring, even supposed that the Geosaurus 

 might be merely the young of that species. But the differences in the shape of the 

 teeth are associated with differences in the structure of the cranium, of the sclerotic, 

 and, what is still more important, in that of the vertebrae themselves, which are sub- 

 biconcave and contracted in the middle of the centrum. With these evidences, 

 therefore, of the importance of the differences indicated by different forms of the 

 teeth of the acrodont Sauria, one may be justified in the expectation that the Leiodon 

 will prove to be a genus alike distinct from both Mosasaurus and Geosaurus, and. 

 as probably tending to fill up the hiatus that divided those genera in the series of 

 Acrodonts, as it was known to Cuvier. 



The additional evidence which has been received in elucidation of this highly 

 interesting family of Saurians, since the publication of my ' Odontography,' has 

 tended to confirm the conclusions stated in that work relative to the Leiodon anceps. 

 The Mosasaurus of the Green-sand Formations in North America, has been satis- 

 factorily shown in Professor Goldfuss's Memoir, to be a species distinct from that of 

 the Cretaceous Deposits at Maestricht. The maxillary teeth show the same generic 

 characters, the two sides being unequal, but with specific modifications. The ptery- 

 goid teeth are ten in number on each pterygoid bone, attached in like manner to an 

 alveolar border, which is convex both downwards and outwards : all the crowns of 

 these pterygoid teeth had been unfortunately broken off and lost. 



Mr. Charlesworth has described and figured in the first part of the 'London 

 Geological Journal,' a portion of jaw-bone, with five teeth, of the Leiodon anceps, which 



