MEBRIAM:'TBIASSIC K'HTIIYOSAI IMA. 81 



in the ichthyosnurian types of Jurassic time. Judging from the number of 

 forms known in the Trias this plasticity was at least as pronounced in that 

 period. 



The plasticity, or the greatest variation, in ichthyosanrs lias appeared in 

 those portions of the skeletal structure which stood in the most immediate rela- 

 tion to their environment, and could be most easily modified by changes in 

 habits, food, or any variation in the elements of their surroundings. The most 

 variable portions of the skeleton are probably the paddles and the caudal fin. 

 In many of the principal characters of the cranial region of the skull a most 

 remarkable conservatism is shown; only in the form of the rostrum and in 

 the nature of the orbit do we find variation comparable to that of the limbs. 

 The significance of the location of the centers of greatest variation seems to be 

 that the variation is to a great extent determined by the peculiar relation of the 

 animal to its environment, and in those centers where no distinctly important re- 

 lation, of this nature exists variation has been insignificant. 



The fact that the different ichthyosaurian forms recognized are not easily 

 separable, offers some support to the view that they represent stages in a dif- 

 ferentiation of the group which has taken place at a fairly uniform rate, and 

 has not therefore given us sharply distinguished and persistent resting stages, 

 such as have sometimes been considered to represent genera and species. 



In so far as the above suggestions hold good, the evolution of these forms 

 may indicate that the species have originated either as the direct result of use 

 and disuse due to pressure of the environment; or that they have arisen as 

 variants or mutants in which the direction of mutation has been determined by 

 the same external factors which would be supposed to direct use and disuse 

 in the application of the Lamarckian hypothesis. According to this suggestion 

 the known ichthyosaurian types may be explained as having arisen under 

 slightly different sets of conditions, either at the same time in different places, 

 or at different times in the same place. 



Whatever 'be the ultimate cause of the progressive specialization seen in the 

 Ichthyosauria, this tendency became almost a constitutional feature. In spite 

 of minor variations and of the inclination of geographically separated types to 

 take somewhat different paths in their specialization, the general trend of evolu- 

 tion of all forms over the earth has been in the same direction, and at about the 

 same rate. In most instances the horizon in which a Triassic ichthyosaur belongs 

 may be determined by the degree of specialization of any one of a considerable 

 number of structures, as e.g., the zygapophyses of the vertebrae. Where char- 

 acters of late forms seem to appear in early ones, the average of the characters 

 of the whole organism will be found to indicate a relatively low level of special- 

 ization. In the tail of the Middle Triassic CymboximiHli/lus the reduced upper 

 arches of the distal caudal region suggest a caudal series like that of the later 



