MKKKIAM: TH1ASSIC ICHTHYOSAUBIA. ::', 



show bicipital rib articulation in the dorsal region in one case and unicipital 

 articulation in the other. That forms of articulation as different as these 

 occurred in the same region of the vertebral column in variations of the same 

 specific or even generic type seems improbable. It appears that we are dealing 

 either with different parts of the vertebral column of similar individuals, or 

 with quite different types occurring toge ther, like Torctocnemus and Merriamia 

 in the Upper Triassic of California. 



With some evidence of double articulation in the anterior dorsal region and 

 also in certain posterior dorsals, it is possible that all ribs back to the pelvis 

 were double-headed, and that the vertebrae possessing a single large lateral 

 apophysis, but without facets for lower arches, were in the pelvic region or 

 immediately behind it. 



If it should be shown that figure 62 page 50, really represents a type with 

 unicipital middle dorsal ribs it is probable that there are two types represented, 

 one with mainly or entirely single-headed ribs and the other with double- 

 headed ribs back at least to the pelvis. 



In neither case does there seem to be evidence that there are double rib 

 articulations in the caudal region, behind vertebrae to which single-headed ribs 

 are attached. 



A typical vertebra from anterior portion of the caudal region is repre- 

 sented in figures ]20a, 120?;, and 120c. The centrum shows some lateral 

 compression and has a hexagonal cross-section. The inferior surface presents 

 prominent angles on which distinct facets for the inferior arches are present. 

 Other posterior caudals, one of which shows a high degree of lateral compres- 

 sion, are represented by figures 118 and 119 (Fraas, 1891, Taf. 3, figs. 15& and 

 16&). 



Most of the forms of vertebral centra seen in the specimens described above 

 do not correspond exactly to those of other ichthyosaurians. In those with 

 double-headed rib articulation the heads seem to have been more widely sepa- 

 rated in the posterior dorsal region than in Ichthyosaurus. This may indicate 

 a more primitive stage of a reptilian form originally with double-headed ribs, 

 from which Ichthyosaurus is derived. The centra in which the position in the 

 vertebral series has been determined by the form of the cross-section, and by 

 the confluence of the upper portion of the rib articulation with that for the 

 upper arch, are much like cervicals of Ichthyosaurus, Shastasaurus, or Gym- 

 bospondylus. In the centra with single articulation in the pelvic region the 

 form is not very different from that in Cymbospondylus. If the specimen 

 represented in figure 62, page 50, is a middle dorsal it is different from any other 

 form known, but would apparently approach nearer to Mixosaurus than to any 

 other genus. 



As far as has been observed the centra of all of the vertebrae of the M. ( ?) 



