78 MEMOIRS OF THE UNIVERSITY OP CALIFORNIA. 



Some of the most definite opinions regarding relationship to other groups 

 are those of the several writers who have considered the Rhynchocephalia the 

 ancestral type. While the Ichthyosauria show a strong resemblance to these 

 diaptosaurians in many characters, it is to be questioned whether this similarity 

 is more than a survival of common primitive characters in both. At any rate, if 

 it be shown that a number of the most characteristic features of the Diaptosau- 

 ria are lacking in the Ichthyosauria we shall hardly be justified in considering 

 the diaptosaurs as their ancestors. 



Recently McGregor 1 " has expressed the opinion that the diapsidan order 

 Phytosauria includes the nearest known relatives of the ichthyosaurs, and that 

 they represent the nearest approximation to the amphibious form through 

 which the ichthyosaurs passed in their evolution from rhynchocephalian-like 

 ancestors, "though no knoirn phvtosaurian can actually be the ancestor of the 

 Ichthyopterygia. ' ' 



The phytosaurs and the ichthyosaurs have many characters in common, and 

 particularly the phytosaurs show some affinity with the most primitive diap- 

 tosaurians. There are, however, a number of characters which seem to indi- 

 cate that the phytosaurs and ichthyosaurs have arisen quite independent of 

 each other. In order also to have the appearance of being intermediate between 

 the ichthyosaurs and the primitive diaptosaurs, the phytosaurs should precede 

 the ichthyosaurs in time. We find, however, that the phytosaurs belong main- 

 ly to the latter part of the Upper Triassic, while the ichthyosaurs were a sharp- 

 ly differentiated and highly specialized group early in Middle Triassic time, and 

 had already expressed in them most of the characters which separate their Jur- 

 assic representatives from the phytosaurs. In spite of the fact that the orbits 

 were much smaller in the earlier ichthyosaurs than in the later genera, and that 

 the inferior temporal arcade was large enough so that a primitive lateral open- 

 ing would not necessarily be closed by contraction of the bar, this lateral open- 

 ing is not present as in the Phytosauria. There are also no antorbital and no 

 external mandibular vacuities in the ichthyosaurs; the squamosal and supra- 

 temporal are not fused ; the pleurocentrum of the atlas was large ; no ectoptery- 

 goid is certainly known; a large pineal foramen was present; the exoccipital 

 and opisthotic were not fused ; there was no tendency to develop a secondary 

 palate ; the pterygoids were in contact with the basioccipital ; and the thoraco-" 

 lumbar vertebrae were more numerous than in the phytosaurs. 



The phytosaurs are typically rhynchocephalian in more characters than the 

 ichthyosaurs. The main features in which they show special similarity to the 

 ichthyosaurs are found in the structure of the rostrum and the nasal region. 

 The median position of the nares appears, however, almost a normal character 

 of marine air-breathing vertebrates, and the extension of the premaxillary re- 



s McGregor, J. H., Op. tit, J>. 89. 



