Caprina Limestone T>c<l*. 105 



The writer has repeatedly shown that the stratigraphic posi- 

 tion of the beds was in the middle of the Lower Cretaceous or 

 Comanche series instead of at the top of the Upper, as believed 

 by Uoemer and Shumard, and hence, aside from the paleon- 

 tologic evidence, he would assign these beds to a still lower 

 horizon, probably the Uppermost Neocomian, or Transitional 

 Neocomian-Gault, for the following reasons : 



1. The fauna does not contain a single characteristic genus or 

 species of beds of higher position. 



2. The beds occur immediately beneath the Washita Divis- 

 ion, which contain numerous species resembling those of the 

 (lault of Europe. 



3. The beds bear a remarkable paleontologic and stratigraphic 

 resemblance to the Requienia Limestone beds of France and the 

 Spanish Peninsula, where similar limestones, with Radiolites and 

 Requienia, abound in the Upper Neocomian. 



IV. DESCRIPTION OF SPECIES. 



Ostrea munsoni sp. nov. 



Plate XII. 



Compare 0. Joanx Choffat. Recueil de Monographies Stratigraphiques 

 Sur le Systeme Cretacique du Portugal, par Paul Choifat. Lisbon, 1885, 

 p. 34, plate 1, figs. 1-7. 



Very thin and flat; elongately sub-triangulate and marked 

 by man 3^ well defined radiating ribs; the pallial extremity 

 rounded ; beak more or less acuminate and slightly deflected, 

 and evidently slightly attached ; the inferior valve slightly con- 

 cave, nearly flat, and showing near its beak an area of attach- 

 ment. The larger valve flatly convexed and only slightly 

 larger than the lower; the ornamentation of both valves is sim- 

 ilar, and as remarked by Choffat in his description of Ostrea 

 Joanx a very similar form from Portugal, the two valves present 

 an appearance as if they had been plicated together, the one 

 upon the other. Each shell is very thin, and the living space 

 small. When closed together the thickness of both valves is 

 hardly one-twentieth the length of the shell. 



The finely fluted ribs are slightly sinuous, continuous from 

 beak to base or sometimes bifurcated, alternating with short ribs 

 extending only halfway from base. This is especialty true upon 

 the lower valve. This species is easily distinguishable from all 

 the other North American oysters by its extreme flatness and 

 thinness. 



15 BIOI,. Soc. WASH., VOL. VIII, 1893. 



