PELECYPODA—TELEODESMACEA. 



553 



operculum-like, flat or conical, with central or eccentric umbo. 

 Teeth not forming the hinge but specially modified for the verti- 

 cal motion of the operculum-like valve ; they consist of two 

 vertically striated processes on the smaller valve, fitting into 

 sockets near the outer wall of the fixed (larger) valve ; next to and 

 outside of the sockets are two large un- 

 equal, slightly excavated muscle scars into 

 which fit the two broad projecting muscle 

 plates from the upper valve. Pallial line 

 simple, enclosing the whole cavity. Co- 

 manchic-Cretacic. 



524. R. texanus Roemer. (Fig. 756— 



757.) Comanchic. 



Larger valve marked with twelve sub- 

 equal vertically ridged ribs. 



Upper Fredericksburg (Edwards): Texas. 



525. R. davidsoni Hill. (Fig. 758.) 



Comanchic. 

 More slender than preceding, with more 

 numerous and finer ribs. 



Upper Fredericksburg : Texas. 



526. R. austinensis Roemer. Cretacic. 

 Larger valve made up of successive lami- 

 nae which are bound firmly together by 

 some of the polygonal cells composing 

 each lamina, being so arranged as to form 

 furrows upon the inside and ridges on the 

 outside. These laminae are almost perpen- 

 dicular to the broad funnel-shaped interior 

 of the shell. 



Characteristic of the Austin (Niobrara) of 

 Texas ; and at about the same horizon in San Luis Potosi. 



527. R. maximus Logan. Cretacic. 

 Shell inversely conical. Outer surface of lower valve marked 



by parallel longitudinal ridges composed of overlapping plates ; 

 inner surface smooth. Valve composed of circular plates placed 

 one upon the other. Upper extremity of lower valve resembling 

 a flange bent upward and outward. Height of lower valve 3 to 4 



Fig. 758. Radiolites da- 

 vidsoni. (After Hill.) 



