30 J. C. Merriam — Triassic Ichthyosauria. 



can obtain for use in working out the phylogeny of the Ichthyo- 

 sauri, it can hardly furnish the whole foundation for a definite 

 classification. Inside the American group there seems to be 

 considerable variation, though as yet we do not know all of 

 the most important characters of these forms. Toretocnemus 

 and Merriamia have very similar limbs but differ considerably 

 in the structure of the vertebrae and ribs. It is perhaps a 

 significant fact that of the several genera, Toretocnemus, with 

 the largest vestigial fourth digit, appears in its general struc- 

 ture to be nearest to some of the earliest forms of Europe, rep- 

 resented by Ichthyosaurus (?) atavus* from the lower portion 

 of the middle Trias. 



* Recent comparisons of Ichthyosaurus (?) atavus with the types of Mixo- 

 saurus show that most of the known vertebrae of atavus are quite different 

 from those of the true Mixosaurs. . So far as is known, they appoach most 

 nearly the type of the true Ichthyosaurs. They may belong to Ichthyosaurus, 

 possibly to Toretocnemus, or may represent an undescribed genus. 



University of California, Berkeley. 



