MERRTAM: TRFASSIC ICHTHYOSAITRIA. sr, 



Considered without regard to age, the dividing line in the Ichthyosauria 

 would certainly be drawn between the well-known Triassic genera on the one 

 hand, and on the other hand the post-Triassic Ichthyosaurus and later types. 

 Considered from the historical point of view it might be presumed that as 

 most of the characters in which these groups differ are closely connected with 

 the peculiar kind of adaptation which we see here, the differences simply ex- 

 press stage of evolution; and that probably some of the known Triassic forms 

 are ancestral to Ichthyosaurs, Baptanodon,and.Opthalino8auru8. 



If the post-Triassic types be excluded from consideration, as presumably 

 derived from some of the known Triassic forms, the contrasts shown in the 

 remaining ichthyosaurs are much less marked. The principal contrasts seen 

 in the Triassic forms are in the structure of the podial region of the limbs, in 

 the rib articulation of the cervical and dorsal region, and possibly in the 

 dentition. In Mixosatints the podial region is broad and forms a relatively 

 large part of the paddle. In the American Upper Triassic genera, which are 

 probably slightly younger than Mixosaurus, the corresponding region forms a 

 relatively small portion of the limb. It is also relatively narrow and the 

 number of digits is reduced. In Cymbospondylus, from an earlier Middle 

 Triassic stage than Mixosaurus, the structure of this region is not certainly 

 known, but the podial portion appears to have three transverse rows of pha- 

 langes like Mcrrianiia, while the form of the distal end of the ulna suggests 

 the presence of a fourth or posterior element in the proximal row of the meso- 

 podial region as in Mixosaurns. In the rib articulation, Mixosaurus seems to 

 be characterized by the presence of only a single head in the dorsal and cervical 

 region, and a possible incipient division of the broad heads of the posterior 

 dorsals and anterior eaudals. In all of the American Triassic genera, except- 

 ing Toretocnemus, no division occurs in any part of the series excepting in the 

 the most anterior portion of the vertebral column, where it is always present. 

 In Toretocnemus the middle dorsals show a wide division of the rib head. 

 This seems to extend well back toward the pelvis. The anterior dorsal and 

 cervical regions of this genus are unknown. The limb structure is apparently 

 like that in Merriamia. In the specimens referred to Mixosaurus^} atavus, 

 the oldest known ichthyosaurian occurring in the Middle Trias, both single and 

 double articulations occur, and two distinct types may be represented. 



On the basis of difference of limb structure and rib articulation, Mixosaurus 

 would apparently be separated from all of the American genera as a broad- 

 paddled, pentadactyl type, in which bicipital rib articulation is reduced to a 

 minimum. The American genera including Toretocnemus would be distin- 

 guished as narrow-paddled and with persistent bicipital rib articulation in the 

 cervical region. Toretocnemus might be classed with the American shastasaur- 

 inesasaform in which the limb structure had become narrowed or leptocheirous, 



