.MKKRIA.M: TRIASSIC K'UTI I V( ISACRI A. 83 



sauridae, Baptanodontidae, and Mixosauridae, were established. To these 

 MeiTiam has more recently added a fourth group, the Shastasauridae (1902, 

 2). At the time when these families were described the arrangement of the 

 ichthyosaurian forms seemed fairly simple, Imt in reality coni])aratively little 

 was known regarding any of these groups excepting the Ichthyosauridae. 

 Since that time the principal points in the structure of Mirosanrus have heen 

 made known by Repossi (1902), the material available for study of the Amer- 

 ican Triassic forms has very greatly increased, and large additions have been 

 made to our knowledge of the structure of Bapt<uto(lon" and Oitthalmosaurtix.'- 



With the recent increase in the available information, many points of 

 relationship have been determined among the ichthyosaurian forms, but the 

 tendency on the whole has been to make a satisfactory arrangement more 

 difficult than before. While the four families already described may still be 

 recognized as fairly distinct groups, they do not adapt themselves to a phylo- 

 genic scheme such as would seem to be demanded by the long historic series 

 now known, and in which cognizance is taken of the facts of geographic dis- 

 trilmtion. The evolutionary history of the Ichthyosauria now extends from 

 the Middle Triassic to the Cretaceous, a considerable series of forms being 

 known from all of the principal marine horizons representing this division of 

 geologic time. We find the Ichthyosauria as a group showing a remarkable 

 unity with a fairly direct tendency of evolution marked out in all the subdivi- 

 sions, and a considerable number of stages are passed through in the known his- 

 tory. It is therefore to be expected that to express the classification of the 

 group in the form of phylogeny would be a fairly simple matter. In a gen- 

 eral way the horizontal classification, viz., that recognizing the forms of only 

 one horizon, meets few obstacles. In a linear arrangement the tendency of 

 progression, specialization, or evolution is distinct, and the stages recorded are 

 fairly definite. In viewing the whole scheme both horizontal and linear it is, 

 however, difficult to discover characters which are fundamental and persistent, 

 or which are adaptive and persist unchanged to such a degree that they may 

 be used in a classification scheme. To a certain extent it is true that in pass- 

 ing from one period to another the characters considered as diagnostic are 

 replaced by others. 



The characters on which the classification of the Ichthyosauria was based 

 by Baur included only modifications of the extremities and dentition, both of 

 them features which would most readily respond in adaptation to aquatic con- 

 ditions. The distinguishing characters of the Shastasauridae as originally 

 described were found in the "peculiar articulation of the dorsal ribs, the form 

 of the pelvic arch, and the presence of the long-spined chevrons." It has been 



liilmore, C. W., Mem. Carneg. Mus., vol. 2, no. 2, and no. 9. 

 Andrews, C. W., Geol. Mag., Dec. 5, vol. 4, p. 202. 



