7fi MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



been to develop those features which are of importance in purely aquatic life, 

 and to reduce in importance those structures which have little or no function 

 in such an environment as would be met under aquatic conditions. 



In the earlier stages of the study of the Ichthyosauria various opinions 

 were expressed 28 as to whether the characters showing close relation of these 

 forms to their aquatic environment were survivals from a very primitive fish- 

 like ancestral type, or whether they were secondarily acquired through accom- 

 modation of a land type to life in the sea. Practically all investigators who 

 have expressed themselves on this point in recent years have held to the opinion 

 first given a basis of observed fact by Baur (1887, 1), viz., that the ichthy- 

 osaurs are specialized aquatic forms occupying in the Reptilia a position sim- 

 ilar to that of the cetaceans in the Mammalia. This view receives support 

 amounting to a demonstration from the evidence gathered in a study of the 

 history of the group. Not only is the stage of evolution of the Triassic repre- 

 sentatives nearer the shore or semi-aquatic reptilian type than in the later ones, 

 but a definite and fairly regular gradation or progressive specialization from 

 the earliest forms to the latest seems to be recognizable in many parts of the 

 skeleton. The evidence of gradual modification toward the fish-like type pre- 

 sents itself not only in a comparison of the types of the successive periods 

 with each other, but it is almost as distinctly seen in contrasting the forms 

 of the latest Triassic with those from the middle division or from the lower 

 part of the Upper Trias. It is clearly noticeable again in contrasting the 

 faunas of the Upper and Lower Lias, or in a comparison of the Upper Liassic 

 species with those of the later Jurassic and Cretaceous. Taken as a whole 

 the evidence is of such a nature as to leave no room for doubt as to the course 

 of evolution of the known representatives of the Ichthyosauria. While we 

 are unfortunately not able to see back to the beginning of the evolutionary his- 

 tory, extending from the Middle Trias to Cretaceous, it can hardly lead from 

 any other than a typical reptile with extremities constructed largely, if not 

 entirely, for ambulation. 



In the evolutionary series beginning with the unknown crawling ancestor 

 of the ichthyosaur, Middle Triassic forms of the type of Cymbospondylus rep- 

 resent a stage in which the animals had already abandoned the shore as. a 

 place of regular habitation, but still retained in their skeletal structure enough 

 original characteristics to give a clue to their origin. Contrasted with the 

 Middle Triassic Cymbospondylus type we have at the latter end of the series 

 such forms as the Baptanodon-Opthalmosaurus group in which the adaptation 

 to life in deep water is so extreme as to have suggested to otherwise careful 

 investigators that these forms were less specialized than any other reptiles. 



28 Gegenbaur, C., Untersuchungen zur vergleichenden Anatomie der Wirbelthiere. 2 Heft., 1865. Paddles 

 derived from selachian fins. 



Haeckel, E., Generelle Morphologic der Organismen. Bd. 2, p. 134, 1866. Resemblance to fish secondarily 

 acquired. 



