68 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



of the fundamental forms from which the modified structures seen in the later 

 Ichthyosauria have been derived. Particularly is this true of the podial 

 portion of the limb which is so well shown in the Italian mixosaur specimens. 



Though the suggestion has been made that the most primitive form of limb 

 in the Jurassic Ichthyosauria possesses less than five digits, 2 " it is probable 

 that the pentadactyl form seen in Mixosaurus is at least as near the primitive 

 type as the limb of any known genus. The suggestion that the most primitive 

 ichthyosaurian limbs have less than five digits is based on the supposition that 

 the short first digit has disappeared. As the ichthyosaurian limb was already 

 in Mixosaurus a highly specialized paddle it is to lie presumed that if the first 

 digit were to disappear at all, it would have been eliminated before the mixo- 

 saurian stage of evolution was reached. There are, therefore, the alternative 

 hypotheses that the mixosaur paddle retains the primitive five digits and is 

 the most primitive known, or that after having lost digit number one, it has 

 brought the number of phalangeal rows up to five again by developing a digit 

 not represented in the original five, and is therefore not a particularly primi- 

 tive type. 



In the development of the paddle in some aquatic animals there has been 

 a tendency to eliminate the first digit, but this is by no means always the case. 

 Judging from all the suspected relationships of the Ichthyosauria there is good 

 reason to suppose that the ancestors of this group were forms possessing five 

 digits. If such be the case, the general prirnitiveness of the mixosaurian limb, 

 and the fact that this is the most ancient paddle of which the podial region 

 is known, suggest that the digits seen here are the primitive five phalangeal 

 series. 



Another possibility, which should perhaps be borne in mind, is that the 

 early ancestor possibly had only three or four digits. From what we know 

 of the primitive reptilian groups this seems improbable, but the fact that all 

 of the genera of the American Upper Triassic show a paddle structure of this 

 nature, and that there is reason for suspecting the occurrence of a similar 

 structure in the Middle Triassic Cymbospondylus lends some appearance of 

 reasonableness to this hypothesis. 



The form of the paddle and the arrangement of the mesopodial elements ill 

 Mixosaurus in some ways closely resemble the structure seen in the latipin- 

 nate forms of Ichthyosaurus (figs. 104 and 105). In both cases the paddle 

 is relatively broad, and the third element of the second row of mesopodials 

 articulates proximally upon the intermedium. In the longipinnate forms of 

 Ichthyosaurus (fig. 102), as in the American Upper Triassic genera, the third 

 element of the second row articulates with the posterior element of the first 

 mesopodial row and not at all, or only incidentally, upon the intermedium. 



2 Lydekker, E., Geol. Mag., 1888. Dec. 3, vol. 5, p. 310. 



