§ 38. REPTILES OF THE CHALK. 353 



ancient slate rocks. But from the characters of the fossil 

 fishes, which are all of cretaceous genera, M. Agassiz was 

 enabled to solve the problem, and prove that the Glaris 

 slates belong to the chalk formation ; the altered condition 

 of the rock having resulted from the effect of high tempe- 

 rature under great pressure. 



In concluding this cursory notice of the fishes of the 

 cretaceous epoch I would remark that two-thirds of the 

 genera, and almost all the species, are extinct, but related 

 to tertiary forms. This result is in accordance with that 

 derived from the examination of the zoophytes, mollusca, 

 and other organisms, which have come under our notice in 

 the course of this review. 



38. Reptiles of the chalk. — The remains of reptiles 

 discovered in this formation, though not very numerous, are 

 sufficient to arrest attention, when contrasted with the 

 entire absence of all traces of warm-blooded animals. We 

 perceive, as it were, the first indications of that remarkable 

 discrepancy in the relative numerical proportion of the 

 mammalia and the reptilia, which distinguishes the se- 

 condary from the tertiary and modern epochs. 



The most extraordinary of the reptiles peculiar to the 

 chalk, is the Mosasaurus of Maestricht {ante, p. 311). A 

 few vertebrae, apparently of this animal, have been found 

 in the white chalk near Lewes. In the cretaceous strata of 

 America, my friend Dr. George Morton, of Philadelphia, 

 discovered teeth, which are specifically identical with the 

 Maestricht reptile.* In the Essex chalk, portions of a 

 jaw, with teeth, belonging to a saurian of the same genus, 

 were discovered by Mr. Edward Charlesworth many years 

 since ; the symmetrical form, and smooth surface of the 

 teeth, separate them from the above, and Mr. Charlesworth 

 has named the species Mosasaurus stenodon. 



* Dr. Morton's Synopsis of the Cretaceous Strata of N. America, 

 PI. XI. fig. 9. 



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