J. 0. Merriam — Triassic Ichthyosauria. 23 



Art. III. — The Types of Limb-Structure in the Triassic 

 Ichthyosauria ; by John C. Merriam. 



Introduction. 



Of the numerous valuable contributions to paleontological 

 literature made by the late Professor George Baur, one of the 

 most interesting was that in which he furnished evidence that 

 the limbs of the Jurassic Ichthyosaurs were highly specialized 

 structures developed in adaptation to aquatic life. * In advanc- 

 ing this view he opposed the theory of Gegenbaur and others, 

 who held that they were generalized and intermediate between 

 the pentaclactyle limbs of the higher vertebrates and the many- 

 rayed extremities of the selachians. As was shown by Baur, 

 the limbs of the Triassic Ichthyosaurs come nearer to the type 

 of extremity found in the primitive reptilia than do those of 

 the later representatives of the order. The character of the 

 modification of the limbs, and in fact the whole structure of 

 the body in the later Ichthyosaurs, indicated to him that, as a 

 group, they bore the same relation to the Rhynchocephalia 

 that the cetaceans bear to the primitive mammals. 



At the time Baur wrote on this subject, the only available 

 Triassic specimens showing the limbs were those from the 

 bituminous shales of Besano in Lombarcly. These he separated 

 from Ichthyosaurus as a new genus, Mixosaurus. 



Within the last few years considerable collections obtained 

 from the Triassic of California have brought to light several 

 new groups of Ichthyosaurians differing not a little from the 

 previously known genera of Europe. So many new forms 

 have appeared in this fauna that some of the questions relating 

 to the origin and descent of the Ichthyosauria are reopened. 

 The addition of new material has made the problems more 

 complicated, but it is hoped that before we again reach the 

 limits of profitable discussion it will be possible to add some- 

 thing to our knowledge of the origin and the history of the 

 group. 



Characteristics of known types. 



At the present time we are acquainted with not less than 

 four types of limbs in the Triassic Ichthyosaurs. One of these 

 is represented in Mixosaurus, a second in the genera Toretoc- 

 nemus and Merriamia^ a third in Shastasatirus osmonti and 



* Ueber den Ursprung der Extreniitaten der Ichthyoptergia, Ber. d. xx, 

 Versamml. d. oberrhein. geol. Ver., xx, p. 3. 



f See G. A. Boulenger, Proc. Zool. Soc. Lon., 1904, vol. i, p. 425. Lepto- 

 cheirus Merriam being preoccupied is replaced by Merriamia Boulenger. 



