MKRRIAM: TRIASSIC ICHTHYOSAUEIA. 



117 



short. The upward curvature of the tail as it lies in this specimen may be 

 unnatural, and due simply to movement of the body after death. The down- 

 ward curve of the distal region, being initiated as it is with noticeable modifi- 

 cation in the form of the vertebrae, is evidently natural. Possibly the upward 

 curve anterior to this is also natural, the distinct change in the vertebrae occur- 

 ring at the crest of a long, upward curve as in MLroxd/trux; or the distal region 

 may have bent down from a nearly horizontal anterior region. 



Facets for lower arches are present on the caudal centra in no. 9947, and 

 the inferior side of the centrum is in all cases sharply angular, but chevrons 

 have been seen only in the region back of the downward bend of the tail, 

 where they are very small, though the facets for their reception seem to be 

 larger than in the anterior portion of the tail. 



The nature of the curvature in the caudal region of this specimen is in some 

 respects not unlike that of Ichthyosaurus, as the upper arches are in relation 

 to the centra but little larger than in that genus. The centra themselves are, 

 however, much larger in the region of the bend, and beyond, than in any of the 

 later ichthyosaurs, and the high thin centra doubtless served to broaden the 

 fin somewhat as the high arches have done in Mixosaurus. The fact that the 

 centra were higher suggests that less curvature of the distal caudal region was 

 needed in order to widen the fin than was required in Ichthyosaurus, and that 

 the bend was a gentle upward curve followed by a somewhat sharper down- 

 ward flexure, as in Mixosaurus. 



Following are the measurements of the series of caudal vertebrae in spec- 

 imen 9947. 



Approximate 

 position of 

 vertebrae, 

 measured 



Presence of Height of 



back from diapophy8eg . centrum, 

 femur as 

 situated in 

 the matrix. 



Anteroposterior 



diameter of 



centrum. 



Transverse 



diameter of 



centrum. 



Height and 

 curvature of 

 neural spine. 



Length of 

 chevron. 



41 absent 



48 absent 



53 absent 



a, approximate. 



