Wieland : Plastron of the Protosteginse. 1 .°> 



facts accumulate, that the flippered turtles represent a great complex 

 of forms which have arisen through repeated invasion of the sea in 

 Mesozoic time, it being indeed not improbable that most of the groups 

 most conveniently grouped as marine subfamilies have thus indepen- 

 dently arisen from more or less nearly related genera of land tortoises. 

 The tracing of such independent lines is, however, doubtless rendered 

 difficult, as much by subsequent homoplastic adaptations, as by the 

 imperfections of the record as known. But while we are not yet in a 

 position to absolutely prove such a polyphyly of the Cheloniidae, the 

 general facts in the case of the Protosteginse, their various ear marks 

 suggesting a certain relationship to Dermochelys by way of the Thalas- 

 semyds, together with culmination in the Cretaceous, assuredly suggest 

 independent origin from forms other than the New Jersey Propleurinae 

 as so closely related to the Cheloninse. 



The hypothesis is therefore advanced, in conclusion, that : (a) 

 The marine turtles are distinctly polyphyletic ; that is, various more 

 or less distantly related tortoises have from the Jurassic on repeatedly 

 assumed littoral habits, and developed flippers. (&) Five of these 

 distinct lines of marine turtles are exemplified by ( i ) Dermochelys, 

 (2) the Protosteginse, (3) the Desmatochelydinse, (4) the Cheloninse, 

 (5) Carettochelys insculpta, the Fly River Turtle of New Guinea, a 

 flippered pleurodiran with complete reduction of the horn shields. 

 (V) The Ancestry of Dermochelys and the Protosteginse falls within 

 the Thalassemyds, or Acichelydidse, and the plastron and nuchal also 

 suggest certain affinities between the latter and some ancient form 

 near to the original Trionychid line. 



As correlative to this hypothesis I may add, though somewhat in 

 repetition, that however one may split hairs about the meager evidence 

 as to the nature of the mutations which have resulted in the osteodermal 

 mosaic of Dermochelys, the safe and simple working view is to my 

 mind that his plastron is a turtle plastron, his nuchal a true nuchal, 

 all his other organization likewise testudinate and impossible of homo- 

 plastic origin, and that his ancestors were simply more ancient than 

 those of the Cheloninse, but withal typical tortoises, quite probably 

 falling, as above suggested, within the Thalassemydidse, and prob- 

 ably without an osteodermal mosaic. The epineural ossicles of Toxo- 

 chelys, and the epi marginals of Lyloloma, show well that an osteoder- 

 mal series corresponding to the hornshield system was once far more 

 conspicuous in the turtles than now • and the keels of Dermochelys are 

 in exact correspondence to such a series. 



