G. R. Wieland — On Marine Turtles. 215 



nalia remaining strong to robust. With reference to the first 

 pair of ribs a word remains to be said. It would seem posi- 

 tively certain that in the ancestry of the Protosteginse these 

 ribs were much suppressed, but for the fact that in Progano- 

 chelys, as described by Fraas (5), the first pair of ribs has a 

 wholly unique development. In this, the most ancient turtle 

 known, the first rib is very large, and of peculiar form, the 

 distal end being widely expanded, but in the vertical direction. 

 In Archelon, however, the very slight distal expansion is lat- 

 eral, and there does not appear to be much in common with 

 this terrestrial Pleurodiran from the Keuper that would sug- 

 gest even remote ancestral relationship. 



The return to a primitive condition, as seen in the ribs above 

 noted, has seldom been demonstrated. The only other well- 

 authenticated instance of such return among the vertebrates 

 within my ken is that of the canines of the camels as explained 

 by Dr. J. L. Wortman. In the Eocene camels, the canines 

 are small and incisiform ; in the Oligocene and Miocene forms, 

 these teeth assume an enlarged and normally caniniform pat- 

 tern, while in the Pliocene and modern forms they again take 

 on the primitive Eocene condition. 



Although it does not as yet seem possible to me to correlate 

 the carapacial changes just described directly with those most 

 probably undergone by Dermochelys, it is believed that the 

 example of compensatory skeletal change here recorded must 

 be regarded as a highly interesting one. 



Measurements of Archelon ischyros. 

 (a) The First Bib. 



(From specimen about 12/13 the size of the type.) 



M. 



Greatest length *74 



Circumference, 18-5 cm out from head . _. *225 



Width half-way from head -075 



Width 50 cm from head . _ -09 



Girth " " -23 



Depth of head in the dorso-ventral direction -15 



Least circumference about 30 cm from head *21 



(b) The length of the second, third, fourth, fifth, and sixth 

 ribs, respectively, in the type specimen, is *95, 101, 1*02, 1'02, 

 and 1-01 meters. (The first rib would be 80 cm long.) 



(c) The width ot the first to tenth ribs of the type taken at 

 their middle point is, respectively, ('08), '075, -075, -078, '015, '07, 

 •065, -06, -055, and '05 meters. 



II.— Associated fossils. — With Archelon ischyros and Marshii 

 there occurs in the uppermost 100 feet of the Fort Pierre (No. 4 

 Upper Cretaceous), as developed on the Cheyenne River, a series 



