M: TRTASSTO ICHTHYOSAURIA. ">7 



be incompletely surrounded by a broad, recurved book formed by the much 

 expanded median side of the blade as in S1i<ist(i*<ittrux. In Del i>liinnx(innix 

 and SlHixInsatn-tiK the pubis is much expanded distally, and the expanded 

 middle portion curves backward so as to form a very prominent hook. 



The form of the ischium is less variable than that of the other elements. 

 In all cases it is an expanded plate-like element without distinctly marked 

 tuberosities or constrictions. It shows the greatest transverse expansion in 

 Cymbospondylus, in which in some instances the width almost equals the length 

 (see fig. 134, p. 120). The narrowest form is seen in Toretocnemus, though the 

 expansion is considerable even in that genus. 



The character of the pelvis in the Triassic Ichthyosauria much resembles 

 that of the early diaptosaurian reptiles, particularly in the nature of the in- 

 ferior or ischio-pubic portion. The upper portion of the ilium is less robust 

 than in the Protorosauria and Pelycosauria, but not less so than in the Pro- 

 ganosauria, Choristodera, and Rhynchocephalia. The pubis and ischium of 

 Toretocnemus resemble the corresponding bones of Stereosternum (fig. 68) in 

 all but minor details of outline, and so far as can be determined the ilium is 

 also similar. In Procoloplwn (fig. 67) the inferior elements are like those 

 of Toretocnemus, though the form of the pubis is more nearly circular. In 

 Champsosaurus the ischium is similar and the pubis is perforated by an obtu- 

 rator foramen. From the inferior elements of the arch in Palaeohatteria, 

 Toretocnemus, and Cymbospondylus differ mainly in the presence of a closed 

 obturator foramen instead of an obturator notch. Excepting Toretocnemus 

 and Cymbospondylus all of the American Triassic genera resemble Palaeo- 

 hatteria in the general form of the ischium, and in the development on the 

 pubis of a posterio-median hook bordering an obturator notch. Of the known 

 forms, Mixosaurus is the only one in which neither foramen nor notch appears 

 in the pubis, though otherwise the form of the element here differs but little 

 from that in the other genera. 



As has previously been suggested by the writer (1902, 2, p. 86), though the 

 pelvis of the Triassic Ichthyosauria shows some resemblance to that of the 

 Plesiosauria, the form shown here denotes mainly a less complete accommo- 

 dation of the skeleton as a whole to aquatic conditions, than we find in the 

 Jurassic ichthyosaurs. Koken 23 has seemed to imply that the plate-like pelvis 

 of Shastasaurus is not primitive, but represents a peculiar accommodation. 



23 Koken, E., N. Jahrb. f . Min. Geol. Palae. Bd. 1, p. 149, 1906. 



"Das Becken von Ichthyosaurus ist nicht eigentlich angepasst; es durchlauft eine Reihe von Veranderun- 

 gen, welehe seinen Funktions-wert mehr und mehr herabsetzen ; gerade bei Shastasaurus liegt Anpassung vor, 

 wie bei Plesiosauriern, well wahrscheinlich, das geht aus den Eesten der kraftigen Hinterextremitiit hervor, diese 

 ein wichtiges Lokomotionsorgan blieb. " 



The view expressed by Koken is possibly not intended to be materially different from that of the writer as 

 given above; though taken with the context it appears to be a criticism of the writer's suggestion that the form 

 of the pelvis indicates "less complete accommodation of the shastasaurian skeleton to purely aquatic conditions 

 than we find in the Jurassic Ichthyosaurus." 



