MKRRIAM: TRIASSIC ICHTHYOSAURIA. .'57 



chevrons are present from the anterior caudal region at least as far back as 

 the bend in the tail. 



In the ichthyosaurs of the Lower Lias of Europe (fig. 48, p. 43) the hypo- 

 central elements are present, in many cases at least, considerably beyond the 

 bend in the tail. According to Owen 17 they are not present on the terminal 

 twenty or more vertebrae in Ichthyosaurus. In some of the Upper Liassic 

 ichthyosaurs of South Germany, particularly in I. quadriscissus (fig. 49, p. 

 43), lower arches are not present in the region of the tail immediately in front 

 of the bend, and have evidently in some cases disappeared entirely. In 

 Opthalmosaurus, according to Andrews, 18 caudal intercentra appear to be pres- 

 ent. In Baptanodon, according to Knight, 1 " the lower arches are absent. 



The history of the Ichthyosauria seems to give us a pretty definite trend of 

 evolution of the hypocentral elements of the caudal region (figs. 43 to 49). It 

 begins in the Triassic with forms in which the lower arches are of the normal 

 chevron type. In the Lower Liassic ichthyosaurs the inferior union of the short . 

 paired elements has ceased. In the Upper Lias the hypocentra may disappear 

 largely or in part, and in Baptanodon, one of the most recent genera of the 

 order, they seem to have disappeared entirely. 



The gradual modification and final elimination of the caudal hypocentra is 

 undoubtedly to be ascribed to the increased efficiency of the terminal caudal 

 fin. In the earlier ichthyosaurs the whole tail served as a sculling oar, and 

 the elongated inferior arches heightened or broadened the tail. In later forms 

 the flexure of the terminal portion of the caudal series made possible the de- 

 velopment of a broad sculling organ in the region where it was most effective, 

 and the power came to be applied largely in that region. 



In the cervical region of Shastasaurus (fig. 57, p. 48) there are facets for 

 an intercentrum between the third and fourth centra, making a fourth inter- 

 centrum not present in Ichthyosaurus. In this respect Shastasaurus may be 

 considered more primitive than later genera. In the older Cymbospondylus 

 there is, however, no intercentrum between the third and fourth vertebrae 

 (fig. 58). The hypocentral elements of the cervical region are not known to 

 be materially different from those of the Jurassic forms. 



Atlas and Axis. The atlas and axis are distinctly separated in all Triassic 

 ichthyosaurs as far as known. In Cymbospondylus and Shastasaurus the 

 adjacent faces of the atlas and axis are both concave and there is no sugges- 

 tion of fusion as in the later ichthyosaurs. The atlas of Cymbospondylus 

 petrinus (pi. 8 and text fig. 38) has about the same width and height, and about 

 three-fourths the anteroposterior diameter of the axis. The anterior face is 



IT Owen, K., Fosa. Eept. Lias. Form., 1870, p. 92. 



is Andrews, C. W., Geol. Mag., Dec. 5, vol. 5, p. 205. 



10 Knight, W. C., See Gilmore, Mem. Carnegie Mus., vol. 2, p. 107. 



