INTEODTJCTION. ix 



Most of the iisla-remains are important as exhibiting osteological characters which 

 cannot be seen so satisfactorily in crushed specimens preserved in hard rock. 



In this volume only the Ichthyosaurs and the Elasmosaurian Plesiosaurs are dealt 

 \Aith, the Pliosaurs and Crocodiles heing reserved for the second volume. 



The Ichthyosaurs are represented by one genus, Ojohthalmosaurus, of which only a 

 single species, 0. icenicus, Seeley, is here recognised, though the variability of the 

 skeleton is so great that some might be inclined to consider several species to be 

 present. It has, however, been found, from the examination of a very large number of 

 more or less nearly complete skeletons, that the different forms pass into one another, 

 so that no line between this and that can be drawn. Many of the apparent differences 

 are due to the different extent to which ossification has proceeded in individuals of 

 various ages, and others arise from the differing conditions of preservation (presence or 

 absence of compression, &c.). 



Ojildhahnosaurus, which is here regarded as congeneric with Saptanodon * from 

 contemporary or approximately contemporary deposits of the United States, seems 

 in many respects to represent the most highly specialised type of Ichthyosaurian 

 as yet known. It first appears in the Oxford Clay, and it is by no means certain 

 that the genus survived even in the period of the Kimmeridge Clay, although possibly 

 the so-called Iclithyosaiinis entheciodon, in which the teeth are very small, may be a 

 related form. 



It is true that a small species of Ichthyosaur from the Cambridge Greensand has 

 also been referred to the same genus under the name Oplithalmosaurus cantairigiensis 

 by Mr. Lydekker, but very little is known of this animal and the presence of facets for 

 three bones on the distal end of the humerus does not seem sufficient evidence, since 

 this character is not confined to Oj)MJiaImosaunis. A portion of a lower jaw from the 

 Upper Greensand of Warminster has also been referred to this genus on account 

 of the small size of the teeth, but in this case also the evidence seems insufficient. 

 It is certain that the Cretaceous Ichthyosaurs that are at all well known are not 

 related to Oj)JithaImosaunis, this being shown by the great development of the teeth 

 (in /. camjpylodon. Carter, from the Gault and later) or by the structure of the paddles 

 (in I. platydacfylus, Broili, from the Lower Greensand of Hanover). 



* For an exhaustive account of the American species, see C. W. Gilraore, " Osteology of Bajiianodon," 

 'Memoirs of the Carnegie Museum,' vol. ii. (1005) p. 77. 



b 



