1916] Wiman: Marine Triassic Reptile Fauna of Spitzbergen 67 



which moves backwards, and first during the Jura period reaches the 

 tip of the tail, where it becomes stationary, after which the portion 

 of the vertebral column located within the fin begins to shrink. In 

 this view of mine I have also been strengthened by Fraas'" description 

 of the remains of a very young specimen of Ichthyosaurus quadri- 

 scissus Quenst., which in this respect constitutes something between, 

 for instance, Mixosaurus nordcnskioldi and the full-grown /. quadri- 



Fig. 1. — Mixosaurus? natans Mer- Fig. 2. — Mixosaurus nordensJcidldi 



riam. Anterior limb, X % (combined Hulke. Anterior limb, X % (after 



after Merriam and partly restored). Wiman). 



H, humerus ; E , radius ; V, ulna. 



scissus. It then also becomes perfectly natural that the fin-defining 

 downward bend of the tail in the older Middle Triassic M. norden- 

 sJcidldi, lies more forward than in the younger Upper Triassic M. 

 cornalianus, in which the fin ought to have had time to move a little 

 farther back. In Cymb ospon dylus petrinus, also Middle Triassic, 

 the tail-curve lies, according to Merriam, still farther back than in 

 the younger M. cornalianus. This, however, need prove nothing more 

 than that in another quite different tribe of ichthyosaurs the move- 



