Vlll INTRODUCTION". 



Museum, has come to the conclusion that this is one of the cases 

 where an adherence to the rule of priority is not advisable. 



In the Sauropterygia a somewhat greater number of genera are 

 admitted than in the Ichthyopterygia ; the forms which are included 

 by many writers in Plesiosaurus being ranged under four genera, 

 which are mainly defined from the characters of the vertebrae and 

 the pectoral girdle. It has, indeed, been proposed by several writers 

 to divide the forms included in the genus Cimoliosaurns into several 

 genera ; but at least one of these proposed divisions rests on a mis- 

 interpretation, while some of the others do not appear to present 

 well-marked distinctive characters. The forms here included in 

 that genus are readily divisible into two groups distinguished 

 by the characters of the vertebrae: and it is suggested that this 

 distinction may eventually be adopted as a generic one. If, how- 

 ever, this be found advisable, it would be almost obligatory to regard 

 the groups into which the genus Plesiosaurus is divided as likewise 

 of generic importance. Jhe various species of the two groups of 

 Cimoliosaurus differ greatly among themselves in regard to the struc- 

 ture of the limbs, so far as they are known ; and if such differences 

 be regarded as generic, almost every species of which the limbs are 

 known would have to form the type of a genus, while we should be 

 unable to classify at least many of those species in which these 

 appendages are at present unknown. Since the text was in type 

 Mr. A. N. Leeds has forwarded to the writer drawings of the cervical 

 vertebras and pectoral arch of a Cimoliosaurus from the Oxford Clay 

 near Peterborough, which, while agreeing closely with C. plicatus 

 in the structure of the cervical vertebras and the general form of the 

 scapula and coracoid, differs in the absence of a median bony bar 

 connecting the former with the latter. If this be a constant feature 

 of this form it will indicate a well-marked specific character. Since, 

 moreover, the cervical vertebrae of this skeleton (No. 28 in Mr. 

 Leeds's collection) are slightly shorter than those of C. plicatus, 

 there is a considerable presumption that this form will prove speci- 

 fically distinct from the latter, in which event it may be known as 

 C. durobrivensis. 



It is not presumed that the classification here adopted will be 

 final, or will even meet with general acceptation ; but it, at any rate, 

 has the merit of simplicity, and of not overloading tho science with 

 a host of generic terms which, at all events for the present, cannot 

 be fully defined, and which would entail the difficulty of leaving us 



