82 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



Ichthyosaurus, but the vertebral centra of this region in Cynibospondylus offset 

 the beginning specialization of the arches by preserving a remarkably high and 

 compressed or primitive form. 



The tendency which the Ichthyosauria have shown to keep the same general 

 trend of specialization through their whole history, and in all geographic 

 regions, is not contrary to the suggestion advanced above: viz., that the modi- 

 fications introduced in this group usually indicate the bringing about of a closer 

 relation to their environment. The all important feature which distinguishes 

 the Ichthyosauria as an order is their aquatic adaptation. Entering as they 

 have into a realm very different from that in which the immediately ancestral 

 types have lived, the factor which has overshadowed all others has been the 

 necessity of getting a working basis for life in water, particularly with relation 

 to locomotion. The extent of the changes required in the accommodation of 

 all ichthyosaurians, whenever and wherever they have existed, to those simpler 

 features of aquatic life which are the same in all periods and in all places, may 

 be responsible in a large measure for the generally similar trend of evolution. 



In this connection we naturally enquire why it was necessary for the Ich- 

 thyosauria to leave their original environment, for surroundings in which an 

 entirely different equipment was necessary. The only explanation which 

 offers itself suggests a possible change of food habits, through the discovery 

 that certain of the thriving cephalopod groups of the early Mesozoic furnished 

 an abundant and easily obtained food supply. Even in the most favorable 

 light, however, the problem of definite variation in the Ichthyosauria in some 

 of its aspects seems to invite a resort to orthogenesis ; or to the view that there 

 are factors influencing variation which are not as yet understood, and that these 

 tendencies to similar variation may express themselves in related groups for 

 long periods, and in regions widely separated geographically. 



Taking the history of the Ichthyosauria from the earliest stages to the latest, 

 the following statements regarding the group seem to be warranted by the 

 known succession of forms: (1) The group is unquestionably a unit, and no 

 doubt has arisen as to the common origin of the forms included in it. (2) 

 There has been a very considerable advance in specialization extending through 

 a long period. (3) Variation has been greatest in those structures standing 

 in the closest relation to aquatic adaptation. (4) Variation seems generally- 

 to be of the continuous rather than of the saltatory type. (5) Variation is 

 remarkably direct, or takes much the same course in all forms of the same 

 geological age. 



CLASSIFICATION OF THE ICHTHYOSAURIA. 



After some knowledge had been obtained of types distinct from Iclithi/o- 

 saurus, the earliest attempt at classification of the groups included in the 

 Tchthyosauria, was that of Baur (1887, 1), in which three families, the Ichthyo- 



