296 MEMOIRS OF THE CARNEGIE MUSEUM 
IV. Sysremaric Posrrron oF PROTOSTEGA. 
CHELONIODEA (Baur). 
(Superfamily of the Cryptodira.) 
A parieto-squamosal arch ; palatine foramen and free nasals sometimes present, 
(Desmatochelydinee); fourth cervical biconvex, with the centra of the sixth to 
eighth much more modified in recent than in most Cretaceous forms. 
' (A) DeRMOCHELYDID™. 
No descending parietal processes; no palatine foramen ; other cranial and limb 
characters not remote from those of the Cheloniide ; carapace represented by the 
nuchal only, and body enveloped in a leathery hide with an osteodermal mosaic; 
no claws. Genera: Dermochelys, Psephophorus, Hosphargis. 
(B) CHELONITDA. 
Skull with descending processes of parietals, so far as known ; palatine foramen 
sometimes present; vomero-premaxillar union usual but not constant; a normal, 
though often much reduced carapace and plastron ; nuchal with or without articular 
process on under side; claws one, two, or more. 
1. Protostegine (Wieland): No free nasals, and no palatine foramina ; obturator 
foramen small and enclosed by ischio-pubie contact on the median line, as in many — 
land forms; nuchal T-shaped; neuralia and pleuralia thin and investing the ribs 
but slightly ; marginalia usually spiniferous on interior borders; plastral elements 
of medium development with numerous digitations on both outer and inner border 
of the hyo- and hypoplastron ; epiplastron of Trionychoid and Acichelysoid form ; 
body enveloped in a leathery hide (?); claws three or more. 
(a) Protostega Cope. — Cranial elements nearly as in Archelon ; mandibular rami 
coossified ; radial process of humerus strong; front flippers very broad and of a 
more distinctly marine type than those of the Toxochelydine (Toxochelys); centrale 
in contact with carpale 1; hind flippers relatively very large, and tarsal region most 
like that of Dermochelys. Species: P. gigas Cope from the Niobrara Cretaceous of 
western Kansas : 
Ratio of spread of front flippers to length of the dorso-sacral series. .... 3.2 
Ratiovoftront=toybindMilipperssscss sean eer ore aces ace 1.3 
First to third fingers free, clawed. 
First to fourth toes free, clawed. 
