190 FOSSIL TURTLES OF NORTH AMERICA. 



In the first instance it accompanied only plastral bones that were described by the writer. 

 Then Case found it with its lateral wings resting on the hyoplastra. He supposed that it 

 had fallen from the carapace to its position, a not unreasonable conclusion. After this 

 Wieland informed us that he had found it twice in place, resting directly upon the anterior 

 portions of the hyoplastra and beneath numerous other skeletal parts. 



(3) Wieland described a bone as the nuchal which can hardly be anything else. 



(4) In the specimen described here as P. potens the bone in question again occurs. On 

 the visceral surface, near the anterior border, there is, on each side, a deep and broad groove 

 (fig. 246) that must have received another bone. This groove reaches nearly to the midline. 

 It is quite improbable that the first peripheral had a long process that filled this groove. On 

 the other hand, the epiplastron might be expected to lie on the upper side of the entoplastron. 

 The structure of this entoplastron is not far distant from that of Chelydra. In this genus 

 there is a median backwardly directed process from the anterior end of which, on each side, 

 there projects a lateral slender process. Were these lateral processes to become directed 

 at right angles with the median process and to become broader, we would have just such an 

 entoplastron as we have in Protostega. In Chelydra, Cheloma, and Caretta we find that 

 the epiplastra overlap the lateral processes of the entoplastron, just as they seem to have 

 done in Archelon and Protostega. 



As regards the superposition of the T-shaped bone on the hyoplastra of Protostega there is 

 this to be said. The entoplastron appears naturally to join the hyoplastra edge to edge. Often, 

 perhaps usually, the hyoplastra push themselves slightly over the entoplastron anteriorly. 

 It is, however, certain that the posterior spine-like prolongation falls above the hyoplastra. 

 If the entoplastron expanded laterally without suturally joining the hyoplastra, there appears 

 to be no reason why it should not rather pass above than below the latter bones. The epiplastra 

 are missing in all the specimens in which the T-shaped bone accompanies plastral bones. 

 These were, however, probably thin, light bones and could easily be removed from their 

 natural position. Their outer ends, doubtless, were in ligamentous union with the anterior 

 borders of the hyoplastra. 



In Wieland's paper just referred to he describes a bone of Archelon which he regards 

 as the left epiplastron. He holds that this bone was overlapt by the outer end of the T- 

 shaped entoplastron, that certain grooves in the bone were filled by digitations or ridges of 

 the entoplastron, and that the thickened anterior end of the bone projected outward and 

 forward as in the trionychids. The present writer finds it impossible to place the bone in 

 this position. It seems more probable that the bone is the right epiplastron, that the thinner 

 end narrowed to a point and was directed inward on the upper surface of the wing of the 

 entoplastron, while the thickened end was directed backward along the border of the hyo- 

 plastron. 



Genus PROTOSTEGA Cope. 



Premaxillary beak less developt than in Archelon. Maxilla with a rather broad grinding- 

 surface, which extends backward to behind front of orbit. Lower jaw with the rami early 

 co-ossified. Entoplastron T-shaped, with the middle third of the anterior border concave 

 from side to side, the distal ends convex. Radial process of humerus large. 



Type: Protostega gigas Cope. 



Protostega gigas Cope. 

 Figs. 247-253. 



Protostega gigas, COPE, Proc. Amer. Philos. Soc., xn, 1871, pp. 175, 452; Fifth Ann. Report U.S. Geol. 



Surv. Montana, etc., 1871 (1872), pp. 323, 335; Vert. Cret. Form. West, 1875, pp. 48, 102, 256, 



plate ix, figs. 1-7; plates x-xiii; Amer. Naturalist, XII, 1878, p. 137. HAY, Pubs. Field Columb. 



Mus.,Zool.,i, 1895, p. 57, plate iv, v; Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 440. CASE, Jour. 



Morphology, xiv, 1897, p. 21, plates v-vi. WIELAND, Amer. Jour. Sci. (4), ix, 1900, pp. 416, 420, 



figs. 8, 19; Mem. Carnegie Mus., n, 1906, p. 279, plates xxxi-xxxiii, text-fig. I. WILLISTON, Amer. 



Jour. Sci. (4), xin, 1902, p. 276, fig. I. OSBORN, Science, xix, 1904, p. 35. STF.RNBERG, Trans. 



Kansas Acad. Sci., xix, 1905, p. 123. 

 Atlantochelys gigas, DANA, Manual Geol., ed. II, 1875, p. 466. 



