G. R. Wieland—On Marine Turtles. 103 



fied. Neuralia normal so far as yet seen, and without median 

 pits or grooves. . Radial process of humerus large and project- 

 ing. 



Species of Protostega. 



A. Niobrara Cretaceous : — 



1. A medium-sized to large turtle, with a thin cara- 



pace investing one-third the rib lengths, and 

 interior borders of marginals splitting into 

 medium-sized digitations. P. gigas. 



2. A small turtle, with more than half the rib 



lengths expanded and with less reduced plas- 

 tron than the preceding ; the hyo- but not the 

 hypoplastic meeting on the median line ; 

 xiphiplastrals only slightly bowed ; marginalia 

 heavy and without digitation of interior 

 borders. P. advena. 



3. A medium-sized turtle, with a comparatively 



thick carapace investing the proximal half of 

 the ribs; plastral form nearly as in P. gigas 

 but with more numerous digitations and smaller 

 fontanels ; marginal borders serrate rather than 

 smooth ; limb bones relatively short and small. 

 This form has the heaviest shell of any Pro- 

 tostegid. The carapace is little more, and the 

 plastron less reduced than in the Cheloniidae. P. Copei. 



4. A large turtle, with xiphiplastra nearly joined 



on the median line and epiplastral pittings on 



outer anterior projection of hyoplastra. P. potens. 



B. Pierre Cretaceous : — 



5. An immense turtle, with neuralia like P. Copei, 



but humerus without a markedly strong radial 

 process ; marginalia strongly digitate on inte- 

 rior borders. P. (Archelon) Marshii. 



Genus Archelon Wieland, 1896. 6 



Premaxillary beak greatly developed and strongly decurvecl ; 

 crushing surface of upper jaw set far forward and limited to 

 vomero-maxillar region ; lower jaw with rami not coossified 

 until old age. Neuralia greatly reduced, to partly abseut 

 anteriorly, and replaced by epineuralia with a deep median 

 sulcus nearly continuous to the eighth true [underlying] 

 neural ; tenth rib relatively large, free, and extending out to 

 marginalia. Radial process of humerus weak. 



Archelon ischyros from the uppermost Pierre Cretaceous of 

 the valley of the South Fork of the Cheyenne River is the 

 only species. 



