MERRIAM: TRIA8SIC ICIITI I Y< >x.\n;i.\. 7!) 



gion occurs in the development of an elongated snout in most groups of reptiles 

 excepting the Crocodilia and Choristodera ; so that in the case of the Phytosau- 

 ria the peculiar feature is the presence of an elongated snout with median nares 

 in a fresh-water form not otherwise highly specialized. The significance of 

 this parallelism is possibly no more than that the phytosaurs are not necessari- 

 ly crocodilian, rather than that the ichthyosaurs are modified phytosaurs. 



It seems that the characters in which the phytosaurs and ichlliyosaurs dif- 

 fer at least balance the special resemblances which may be common inheri- 

 tances from very early ancestors. It also appears easily possible that the time 

 of origin of the ichthyosaurs was at so early a date that some diaptosaurian 

 characters, as the lateral temporal opening, have never appeared in them. The 

 phytosaurs are not far removed from the typical Diaptosauria. They may 

 have originated early or may have had their origin in the latter part of Middle 

 Triassic time. It is doubtful whether their peculiarities, aside from very prim- 

 itive features which parallel characters in the ichthyosaurs, existed in the 

 phytosaurs as early as the time of origin of the Ichthyosauria. It is not im- 

 probable that the phytosaurs simply parallel to some extent the amphibious 

 type through which the ichthyosaurs must have passed in their evolution, and 

 that they developed as a distinct phytosaurian group after the ichthyosaurs 

 had become typical marine animals. 



VARIATION IN THE ICHTHYOSAUKIA. 



The Ichthyosauria exhibit through the whole range of the order such a con- 

 stancy in the major features of their skeletal plan that no question has ever 

 arisen as to whether a particular form is properly considered as a member of 

 the group. There is, however, a considerable range of variation in certain 

 important groups of characters, and indefiniteness rather than fixity of generic 

 and specific features seems to be the rule in the better known forms. Though 

 the variability is shown to some extent in all parts of the skeleton it appears 

 in its most extreme form in those structures which are most distinctly modified 

 in adaptation to aquatic life. 



An example is seen in the paddles of Ichthyosaurus, the fundamental struc- 

 ture of which has always remained much the same. In the podial region the 

 digits have always held close together, in contrast to the divergent fingers of 

 the mosasaurian type; and the phalanges have increased greatly in number, 

 with accompanying average shortening. The number of digits has however 

 varied between fclithi/oMiiu-ux connnioiix with a possible ten longitudinal rows 

 of phalanges, and /. arntirostris with three, or Shastnsaurus probably with 

 two and a rudimentary third. With the variation in the number of digits, the 



