Febeuaby 26, 1909] 



SCIENCE 



341 



DR. WILLISTON ON " THE FOSSIL TURTLES OF 

 NORTH AMERICA" 



In Science of December 4, 1908, Dr. S. W. 

 Williston has published a review of my book, 

 " The Fossil Turtles of North America." 

 This review showed as much appreciation of 

 the merits of the book, and as much leniency 

 towards its shortcomings, as its author could 

 J> desire. However, one or two matters referred 

 to by Dr. Williston seem to require notice; 

 and this is here respectfully presented. 



Dr. Williston takes issue with my division 

 of the turtles into the Athecffi and the 

 Thecophora. To this I will say that unless 

 the early turtles separated primarily into the 

 ancestors of the two groups mentioned, these 

 two suborders are not at all worthy of recog- 

 nition. 



As to the resemblances between the leather- 

 back and the other sea-turtles, I will, to il- 

 lustrate, consider the flippers. The limbs of 

 the primitive turtles were, I believe, not 

 greatly different from those of the snapper, 

 for example. Inasmuch as the ancestors of the 

 leather-back and those of the other sea-turtles 

 started with the same form of fore limb and 

 on entering the water employed this limb in 

 the same way, it is not strange that their 

 descendants have closely similar flippers. 

 The fore limbs of the Trionychidaa have cer- 

 tainly been independently developed, and yet 

 they are being modified in the same direction. 

 Those of Carettochelys are another example. 

 If a thoroughly aquatic pleurodire shall ever 

 be discovered it will doubtless have similar 

 fore limbs. 



As regards the two dermal coverings of the 

 primitive turtles, I may remark that the early 

 reptiles probably had as great need of armor 

 as their modern descendants. A modern croc- 

 odile seems to have need of only a single layer 

 of dermal bones, the so-called abdominal ribs, 

 in the lower wall of its belly; but a cayman 

 requires, in addition to this, an armor of 

 closely jointed bony scutes. The leather-back 

 itself actually possesses the two bony cover- 

 ings in question. Ventrally the plastron of 

 dermal bones is covered by rows of osseous 

 scutes, while on the upper surface the nuchal 



bone is found to be overlaid by a portion o£ 

 the mosaic-like bony armor. Who is to say 

 that never were there under this mosaic of the 

 upper surface also peripheral bones, and even 

 costal plates ? 



Dr. Williston is certainly correct in holding 

 that the elements that I have called fascia 

 bones are of dermal origin. The dermal bones 

 that had sunken beneath the skin I thus 

 named, in order to distinguish them from a 

 more superficial stratum. 



As evidences for the existence of two dermal 

 bony coverings in the primitive turtles I pre- 

 sent for consideration the fact that the 

 leather-back has as many longitudinal zones 

 of bony scutes as the most generalized turtles 

 have of horny shields, and the additional fact 

 that the horny shields do not coincide with the 

 bones which they overlie, but break joint with 

 them.' 



Dr. Williston says that " Toxochelys pos- 

 sesses neural ossicles, while the nearly related 

 and less aquatic Porthochelys is without 

 them." In like manner, the alligator snapper 

 possesses a series of supramarginal horny 

 shields, an ancient inheritance, while the com- 

 mon snapper has lost them. Porthochelys re- 

 tained the nasal bones, but Toxochelys did 

 not. 



My friendly reviewer is in error when he 

 states that I hold that in Toxochelys there are 

 only vestiges of the inner layer of dermal 

 bones. The nuchal, the costal plates (except 

 that part belonging to the true ribs), the 

 peripheral bones, and the whole plastron, be- 

 long to the inner layer. 



Dr. Williston mentions the fact that I have 

 omitted mention of two turtles that have got 

 into literature. One of these is from the 

 Dakota sandstone of Kansas, a formation that 

 has furnished no other remains of the order. 

 This specimen'' is the cast of the cavity of the 

 shell, with indications of some of the ribs. 

 The relationships of the turtle are indetermin- 

 able and fortunately no name has been given 

 the specimen. The other turtle referred to 

 was originally briefly described by Dr. Willis- 



^ See American Waturalist, XXXII., 1898, pp. 

 929-948. 



'Trans. Acad. 8ci., XVI., 1899, p. 67, pi. iv. 



