144 MAE1NE EEPTILES OF THE OXFOED CLAY. 



described from a skull consisting partly of a skull of Metriorhynchus and partly of one 

 of Steneosaurus, a circumstance which has led to much confusion in the use of these 

 names. In 1867 the genus was defined in detail by E. E. Deslongchamps, the species 

 regarded by him as typical being Metriorhynchus superciliosum, Blainv., sp. 



E. Fraas, in his paper on the Thalattosuchia *, distributes the different species of 

 Metriorhynchus into three groups as follows : — 



1. Forms with very short snout, the nasals reaching the premaxillae; prefrontals 



very large ; teeth strongly developed. — M. Irachyrhynchus. 



2. Forms with thick-set snout, but the nasals separated from the premaxillae by a 



considerable interval ; prefrontals moderately large ; powerful dentition with 

 less than twenty-five teeth in the upper jaw. — M. hastifer. 



3. Forms with elongated snout, the nasals in spite of their length being separated 



by a long interval from the premaxillae ; prefrontals smaller and not very 



prominent ; dentition consisting of rather slender teeth, of which there are 



more than twenty-five on each side of the jaws. — M. superciliosum, M. moreli, 



M. blainvillei. 



These groups, which are probably rightly considered by Fraas to be directly ancestral 



to the various forms of Geosaurus and Dacosaurics of higher horizons, hold good in a 



general way for the species now described, though some of these might be regarded as 



breaking down the sharp distinctions between them. Moreover, probably some of the 



new species are more directly in the line of descent of some of the species of Geosaurus 



than those previously known. 



In the present volume seven species of Metriorhynchus are recognised, of which three 

 are new. The skulls range in form from a long slender type to the stout broad type 

 which has been referred to Suchodus clurobrivensis, but the transition from the one 

 form to the other is so gradual that it seems best to retain the same generic name 

 for all. 



The occurrence of so many species of one genus at one locality and on the same 

 horizon is remarkable, but perhaps this is due to the fact that these reptiles were 

 widely ranging pelagic forms which, as in the case of the smaller Cetacea, may have 

 been represented by many species in one marine area of distribution. 



Skull (Pis. IX.-XIII. ; text-figs. 55-59).— The following general account of the 

 skull in this genus is founded largely on the two skulls (R. 3699-3700) of Metrio- 

 rhynchus Irachyrhynchus (PI. XII.) in which the palatal region is fairly well preserved, 

 as described by Mr. E. Thurlow Leeds (Quart. Journ. Geol. Soc. vol. lxiv. (1908) p. 345). 

 Other specimens are referred to when they throw light en points not determinable 

 from these specimens. Unfortunately nearly all the skulls in the collection have 

 been greatly crushed, so that in most cases the roof is flattened and driven down, 



* Palaeontographica, vol. xlix. (1902) p. 1. 



