G. R. Wieland — On Marine Turtles. Ill 



parator ; nevertheless, the species represents an important 

 fossil type. 



A reexamination, of the specimen confirms the characters 

 given, namely, a relatively short humerus and great thickness 

 of the plastron, the latter being half as thick again as that of 

 Archelon. As it is probable that the present turtle was not 

 quite as large as A. ischyros, type (3*4 m.=ll feet long), 

 however, it may be that its plastron was proportionally twice 

 as thick as that of the latter species. 



A fine third marginal of the right side shown in figure 5 is 

 also present, and with it are articulated the keels of the more 

 fragmentary fourth and fifth marginals. These elements are 

 of much the same form as in A. ischyros, type, and although 

 relatively heavier than in that species do not show the great 

 disparity in weight noted in the plastron. More obvious differ- 

 ences of taxonomic bearing, however, are exhibited by a frag- 

 mentary, though otherwise finely conserved neural from near 

 the middle of the neural series. This lacks the groove so 

 highly characteristic of Archelon, and has a strong and continu- 

 ous median ridge precisely like that in Protostega Copei. It 

 therefore becomes necessary to transfer the present species 

 from the genus Archelon to Protostega, where it holds a posi- 

 tion of importance, as exhibiting not only the continuation of 

 the latter genus from the Niobrara into the Fort Pierre, with 

 marked increase in size, but, as far as we know, represents the 

 closest structural approach of the genus Protostega to Archelon, 

 thus far observed. 



The Mounted type of Archelon ischyros (with Figures 6-12, and 



Plates II-IV). 



All the material thus far referred to the genus Archelon has 

 been discovered and collected by myself during the past four- 

 teen years. The original type of Archelon ischyros was found 

 in the brakes of the south fork of the Cheyenne Kiver, about 

 five miles west of the mouth of Rapid Creek, Custer county, 

 South Dakota, in August, 1895. Though a remarkably com- 

 plete fossil, it lacked the skull, which, however, was supplied 

 by an excellent younger specimen with a fine cranium and the 

 lower jaw in place, obtained in 1897. This is here shown by 

 the photographic drawing, figure 6. In 1898, the related type 

 Protostega {Archelon) Marshii was procured from the same 

 horizon as the specimens of Archelon, but on the east bank of 

 the Cheyenne, in the Pine Ridge Indian reservation. Later 

 still, in 1902, a large individual of A. ischyros, nearly identical 

 in size with the original type, was collected at a point several 

 miles farther south, on the west bank of the Cheyenne. This 



