ON CERTAIN HOMOPLASTIC CHARACTERS IN AQUATIC 

 AIR-BREATHING VERTEBRATES. 



BY S. W. WILLISTON. 



In the endeavor to reach more satisfactory conclusions re- 

 garding the life habits of the plesiosaurs, the following article 

 was written nearly a year ago, since which time I have received 

 the very interesting and valuable paper by Professor Fraas on 

 the thalattosuchian crocodiles, 1 in which I was pleased to find 

 a similar discussion, in large part anticipating what I have to 

 say here. I venture to publish my observations as extending 

 and corroborating his statements, and have added further de- 

 tails gathered from his paper — details bearing materially on 

 the subject-matter. 



Among living and extinct vertebrates there are at least six 

 more or less unrelated groups which have become in part or 

 entirely adapted from terrestrial to aquatic existence — the 

 cetaceans, sirenians, ichthyosaurs, plesiosaurs, and certain 

 crocodiles, to which might be added certain testudinates and 

 rhynchocephalians. The pinniped carnivores show a much less 

 complete transformation from the terrestrial carnassidentia, and 

 are of interest here chiefly by reason of the modifications which 

 the limbs have undergone. 



The many structural and phylogenetic differences which these 

 diverse orders present render their many "convergent' or 

 homoplastic features of peculiar interest. These features may 

 be for the most part explained by a progressive adaptation to 

 environment. Wherever food habits and environment have 

 concurred the homomorphic results have often been striking. 

 In other forms, however, where the food habits have been 

 dissimilar, modifications have occurred. And it is in these 

 modifications that I have sought some hints as to the probable 

 habits of the plesiosaurs. The more important adaptive char- 

 acters of these air-breathing vertebrates seem to be the follow- 

 ing. Of course they are not all concomitant, but they are all, 



1. Paleontographica, xlix, p. 3, 1902. 



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