20 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



The distribution of the Triassic ichthyosaurs, ranging as it docs from Europe 

 and Spitzbergen to western North America and possibly to New Zealand, ap- 

 parently indicates that these forms were in existence as a marine type for a con- 

 siderable period even antecedent to Middle Triassic time. As early as the Mid- 

 dle Triassic, forms showing a considerable degree of specialization were present 

 in western Europe and in western North America. Even though a wide range 

 of open sea between these two provinces be presumed to have existed, such a 

 wide distribution as is seen here could not have been accomplished in less than a 

 considerable part of an epoch. 



The wide distribution of the Ichthyosauria in Middle Triassic time with the 

 absence of any known remains from the Lower Triassic makes it difficult to de- 

 termine even approximately the place of origin of the group. 



GENERAL SKELETAL STRUCTURE OF THE TRIASSIC 



ICHTHYOSAURS. 



Though there is much variation in the structure of the ichthyosaurians 

 known from the Triassic, and there is good reason for considering them as di- 

 visible into several natural subgroups, it is also true that they are evidently a 

 unit or a distinct group, the greater part of the evolution or differentiation of 

 which from the primitive type has taken place in Triassic time. It has there- 

 fore seemed important for purposes of comparison to consider the general struc- 

 ture of all the known forms together. In another portion of this paper the 

 characteristics of the subgroups are discussed separately. 



SKULL. 



In all Ichthyosauria, whether from the Triassic or from later periods, there 

 is a remarkable similarity of general characters in the structure of the skull. 

 Though considerable differences appear in a comparison of all the known forms, 

 particularly in contrasting the oldest genera with the most recent ones, these 

 differences consist in the main of variations in the proportions of parts, and al- 

 most no radical changes of characters such as may occur in other parts of the 

 skeleton are found here. The modifications or variations which present them- ' 

 selves during the known history of the group are in the main closely related to 

 variations in the length of the elements of the facial region, or to change in the 

 size of the orbits. The varying characters seem to be almost entirely of the 

 adaptive type, and indicate a close relationship between the evolutionary 

 changes in the group and the greatest needs of the animal under the particular 

 conditions obtaining in its habitat. 



The structure of the skull is well shown in the Italian mixosaurs, but as yet 



