Caprina Limrfitone Be<h. 105 



The writer has repeatedly shown that the stratigraphic posi- 

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

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

 1)y Roemer and Shumard, and hence, aside from the jtaleon- 

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

 horizon, proliably 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 

 Gault 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 Radiolitcs ixnd 

 Requienia.. abound in the Upper Neocomian. 



IV. — ^Description of Species. 



Ostrea munsoni sp. nov. 



Plate XII. 



Compare 0. Joanse ChofFat. Recueil de Monographies Stratigraphiqiies 

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

 p. ;->4, plate 1, figs. 1-7. 



Very thin and flat; elongately sub-tr-iangulate and marked 

 liy many 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 l)oth valves is sim- 

 iUir, and as remarked 1)y Chofifat in liis descri])tion of Ostrea 

 JoaUcT a very similar form from I'ortugal, the two valves present 

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

 ui)on the other. Each shell is very thin, and the living space 

 small. When closed together the thickness of 1)oth valves is 

 hardly one-twentieth the length of the shell. 



The finely fluted ribs are slightly sinuous, continuous from 

 Ijeak to l)ase or sometimes 1)ifurcated, alternating with short ribs 

 extending only halfway from base. This is especialh'- true upon 

 the lower valve. This species is easily distinguishable from all 

 the other North American oysters 1)y its extreme flatness and 

 thinness. 



l.j— Biol. Soc. Wash., Vol. VI II, 1S0:5. 



