INTRODUCTION. 



In a series of short papers appearing at irregular intervals since 1895, the 

 writer has attempted to present some of the more important facts relating to 

 the structure and occurrence of certain of the American Triassic Ichthyo- 

 sauria. In the earlier stages of this work the known material was scanty and 

 generally fragmentary, and it has not been possible in every case satisfactorily 

 to determine the structure or the exact relationships of forms described. With 

 a great increase in the quantity of material in the last few years, it seems 

 advisable to bring together all available information relating to the American 

 genera, and to consider this body of fact in relation to what is known of the 

 earliest ichthyosaurs of other regions, as also in relation to the general problem 

 of the evolution of this peculiar reptilian order. 



Some of the questions here discussed might be considered to better advan- 

 tage by workers more intimately acquainted with the extensive collections of 

 European ichthyosaur material. The writer has, however, been emboldened to 

 attempt generalizations bearing on the work of other investigators in this field, 

 after consideration of the fact that the West- American collections at the pres- 

 ent time seem to represent the largest part of the known Triassic material, and 

 that as yet no other investigator has examined both the European and the 

 American collections. 



The investigation has been considerably delayed by the discovery of a new 

 group of marine reptiles, the Thalattosauria, which it was necessary to under- 

 stand before the true ichthyosaurian remains could be satisfactorily separated 

 for study. Continued discoveries of ichthyosaurian material have also made 

 it advisable to postpone publication until the new specimens could be examined. 

 Even in its present stage the paper here presented can be considered as no 

 more than a report of progress, as new material and additional information 

 regarding the structure and affinities of the Triassic forms are constantly being 

 obtained. 



The material upon which the studies of the American Triassic ichthyosaurs 

 are based includes over fifty specimens, each of which represents a consider- 

 able part of the skeleton ; also a large quantity of more fragmentary material. 

 These collections have been brought together by ten expeditions sent out by 

 the University of California between 1901 and 1907. The material now at 

 hand has been accumulated and made available for scientific study largely 



