.MKKUTAM: TT7TASSK 1 ICHTHYOSATJBIA. 81 



Mixosaurus may lead directly toward Ichthyosaurus, though the American 

 forms would still remain as a separate group, excepting possibly Torcfociicmus. 

 This last genus, in spite of the presence of what is considered a primitive 

 character, is geographically so situated as to make its relationship to Ichthyo- 

 saurus very doubtful', and the similarity of its limb structure to that of Mcr- 

 riann'a makes it probable that it is to be considered as a persistently primitive 

 form coming from the same stock as Mcrrnuuia. 



If the double rib articulation be primitive, there is as yet no form known 

 which would be considered as the ancestral ichthyosaurian type, unless it be 

 one of the imperfectly known forms of J/.(?) atavus of Europe. If for no 

 other reason, the geographic situation of this type, which is itself a factor in 

 classification, is such as to suggest that Ichthyosaurus is descended from it, 

 and that Mixosaurus is also derived from it. Whether the American genera 

 can be considered as derived from If. (I) atavus or from some earlier form 

 must be left undetermined until more is known of 3/.(?) atavus. As yet the 

 available material is very scanty. The geographic situation of M. ( ?) atavus 

 is also very distant from that of the oldest American Middle Triassic forms, 

 and the disparity in age is not great; so that the ancestors of the American 

 forms may have come from some region other than Europe. 



AVithout being able to construct a thoroughly satisfactory phylogenic scheme, 

 there appears to be good reason for considering that there are present in 

 the known material representing this order, at least four groups in which the 

 common origin of the constituent members is fairly certain, and in which the 

 rank of the larger included groups is high enough so that the four divisions 

 must themselves be considered as not ranking lower than subfamilies. These 

 divisions are the Ichthyosauridae, Baptanodontidae, and Mixosauridae of Baur, 

 and the Shastasauridae of Merriam. Of these groups the Ichthyosauridae 

 and Baptanodontidae are both types with bicipital articulation of the dorsal 

 ribs. They are otherwise closely related, and may be brought together as one 

 group, which might well be designated the Ichthyosauridae with two divisions. 

 If Ichthyosaurus is divided into several genera as is the present tendency of 

 writers, and if Baptanodon is to be distinguished generically from Opthalmo- 

 saurus, this family should be subdivided into two subfamilies, the Ichthyosau- 

 rinae and Baptanodontinae. 



The relation of the American shastasaurs to the mixosaur group is not entirely 

 clear, but the character of the dorsal rib articulation and the general correspond- 

 ence in other parts of the skeleton suggest that the resemblance may mean more 

 than primitiveness alone, and that these two divisions should be united in a larger 

 group of family rank, the Mixosauridae with the Shastasaurinae and Mixosaur- 

 inae as subdivisions. 



The following arrangement seems most nearly to represent the relations of 

 these groups: 



