J. 0. Merriam — Triassic Ichthyosauria. 27 



short epipodial elements, there are two groups. Of these the 

 Longipinnati (fig. 5) have an essentially linear mesopodial 

 region, a notched radius and usually three to five digits. The 

 Latipinnati (fig. 6) have an alternate arrangement of the meso- 

 podial region caused by the articulation of two or more distal 

 elements on the intermedium ; there are generally between five 

 and ten digits ; and notches are rarely present on the phalanges, 

 while never on the radius. Compared with these three types 

 the Triassic forms show an unexpected degree of differentiation. 



Primitive characters. 



In spite of the differentiation shown in the Triassic types, 

 they have all retained certain primitive characters not common 

 in the later forms. All show a separation of the radius and 

 ulna, and in all excepting Shastctsaitrus osmonti these elements 

 are elongated and the radius is constricted or shafted. The 

 presence of these and other primitive characters in so many 

 otherwise different forms furnishes us with much stronger 

 evidence of the origin of the Ichthyosauria from generalized 

 shore forms than could have been given by the single type 

 known to Baur. 



Lines of descent. 



A comparison of the Jurassic and Triassic genera of Ichthyo- 

 saurs with a view to determining the lines of descent shows 

 immediately that no known Jurassic form can be considered as 

 having descended from the specialized Shastasaurus. So far 

 as we now know, this group disappeared in the Triassic. Bap- 

 tanodon and Opthalmosaurus are also practically excluded from 

 any comparison with the Triassic genera, as they are compara- 

 tively late forms and could be derived from the latipinnate 

 Ichthyosaurs as easily as from any of the much older Triassic 

 types.* 



The views which we hold concerning the descent of the 

 remaining Jurassic groups involve our interpretation of the 

 homologies of the elements in the Ichthyosaurian paddle. On 

 this subject a considerable variety of opinion has been expressed, 

 particularly with reference to the relationships of the mesopo- 

 dial elements. 



Some years ago Lydekkerf suggested that the most general- 

 ized type of limb in the Jurassic Ichthyosaurs is found in the 



*Dr. 0. P. Hay (Bull. 179 U. S. G. S., p. 463) has, I believe inadvertently, 

 placed the West-American Triassic Ichthyosauria under the Baptonodontidae. 

 Based on this suggastion, Boulenger (loc. cit.) has indicated the descent of 

 Opthalmosaurus from Shastasaurus. The writer is obliged to regard these 

 groups as probably the most widely separated of all the known Ichthyosauria. 



f Geol. Mag., 1888, Decade 3, v, p. 310. 



