58 



MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



74 



75 



The question as to what constitutes a primitive character is not always 

 readily determined, but it would seem that in this case there could be no mis- 

 understanding. The course of evolution of the Ichthyosauria has led quite 

 definitely up to a form in which the principal motive power came to be ex- 

 erted through the caudal fin, and as the tail became more powerful the hind 

 limbs were reduced in size and strength until their function amounted almost 



to nothing. In all of the recognized divi- 

 sions of the Ichthyosauria, even in the spec- 

 ialized Shastasaurinae, the tendency to de- 

 velopment of a powerful sculling tail seems 

 to have been the same, and the ultimate re- 

 sult of evolution if continued long enough 

 must have been the reduction of the hind 

 limbs. In the course of this reduction of the 

 limbs, the arches have been reduced, until in 

 the latest type the rudimentary ischium and 

 pubis are fused. Inasmuch as evolution of 

 the particular kind seen in the ichthyosaurs 

 leads to great reduction or loss of the pelvis, 

 a posterior arch in which the elements have 

 approximately the form and relative size 

 seen in the primitive reptiles, and are associated with a large limb, may well 

 be said to be more primitive than that of Ichthyosaurus. 



In the course of the reduction of the arch in the various divisions of the 

 Ichthyosauria, it is to be expected that slightly varying forms would be as- 

 sumed. That some of the variations which were in vise while the posterior limb 

 was large and functional might represent some slight phase of special adapt- 

 ation is true, though the contention of the writer as previously expressed has 

 not been that the pelvis itself was more primitive, but that it indicated less 

 complete accommodation of the skeleton as a whole to purely aquatic condi- 

 tions than we find in the Jurassic Ichthyosaurus. That the pelvis considered 

 by itself might be assumed to be really more primitive than in the later Ich- 

 thyosauria appears also to be true, when we consider the extent to which the 

 pelvic arches of all Triassic genera resemble the type of pelvis seen in most of 

 the early Diaptosauria. 



Pectoral Arch. The shoulder girdles of the earlier ichthyosaurs resemble 

 that of Ichthyosaurus in strength and compactness, but differ generally in the 

 form of the individual elements. The only bone which is known to be con- 

 stantly different is the scapula. This is always much expanded distally com- 

 pared with the form in all post-Triassic species. The interclavicle, which is 

 certainly known only from Mixosaurus, is nearly triangular, instead of T- 



Fig. 74. Ichthyosaurus communis Conybeare. 



Pelvis, X % Pu, pubis ; Is, ischium ; //, 



ilium. (After Woodward.) 

 Fig. 75. Opthalmosaurus icenicus Seeley. Is- 



chio-pubic bone, about % natural size. Pu, 



pubis; Is, ischium; O.f., obturator foramen. 



(After Andrews.) 



