BRITISH FOSSIL REPTILKS. 125 



In reviewing the principal facts which I have endeavoured 

 to state succinctly in the foregoing pages, the first circumstance 

 that may arrest the attention is the superior number of species 

 which belong to the genus Plesiosaurus, as compared with the 

 genus Ichthyosaurus ; and since the circumstances which have 

 led to the discovery and collection of the fossils of the two 

 genera cannot be supposed to have materially differed, it may 

 be concluded that the Plesiosaurs were rifer in the ancient 

 seas of the secondary epoch, and manifested their typical struc- 

 ture under a greater variety of modifications than their more 

 powerful and destructive congeners. To the four species of 

 Plesiosaurus recognizably defined and described under the 

 names of dolichodeirus, macrocephalus, recentior, and tria- 

 tarsostmiis, or Hawkinsii, I have been able to add descrip- 

 tions or indications of twelve additional species ; the number of 

 Plesiosauri, of which remains have been discovered in the 

 secondary strata of Great Britain, amounting now to sixteen. 



To the species of Ichthyosauri, which Mr. Conybeare has 

 so well defined under the names communis, intermedins, pla- 

 tyodon, and tenuirostris, a very great proportion of the fossils 

 which I have examined are unquestionably referable, the small 

 remainder affording evidence of only the six species indicated 

 under the names of lonchiodon, acutirostris, latifrons, latima- 

 nus, thyreospondylus, and trigonus. 



The deviations from the typical structure of the genus as 

 exhibited in the common species called Ich. intermedins, M'^hich 

 these additional evidences present, are of small amount, the 

 chief being a greater proportional developement of the pectoral 

 arch and its appended extremities, which characterizes the Ich. 

 latimanus. In the other and better known species the prin- 

 cipal modifications of the Ichthyosaurian type are manifested 

 in the magnitude of the entire animal, the proportional deve- 

 lopement of the head, the comparative length and slenderness 

 of the snout, the proportional sizes of the fore and hind pad- 

 dles, and the shape and number of the ossicles composing 

 them. In the forms of the vertebrte there are but slight dif- 

 ferences, and scarcely any in the proportions of the different 

 regions of the vertebral column. 



The part of the skeleton of the genus Plesiosaurus, which 

 has been subject to the greatest extent of modification, is the 

 cervical region, which becomes shorter and stronger as the 

 head increases in size; but the general shape of the head 

 appears to have presented less variety in the Plesiosaurs than 

 in the Ichthyosaurs. The modifications of the vertebrae in the 

 Plesiosaurs are many, though none are of very great extent. 

 The differences of form which the bones of the pectoral and 



