92 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



of the ligament, and the position of the cardinal axis on the cardinal plate. 

 Note that subnacreous layer is composed of distinct major calcite layers 

 (lamellae) parallel to the surface of the shell; each layer is composed of 

 fine fibrous-appearing calcite crystals or plates inclined to the plane of the 

 major layers, and in many cases, with opposed inclination within or 

 between major layers. A hypotype from sands of upper Eagle Ford age, 

 Grayson County, Tex. (locality 50, U.S.G.S. 14553) ; U.S.N.M. 132284. 



Lopha liigubris (Conrad) 



Fig. 12. Thin section (X8) through part of the left valve, showing major 

 calcite lamellae in subnacreous layer and minor inclined fibrous calcite 

 crystals or plates within them, as described for figure 11. A hypotype 

 from New Mexico (locality 1, U.S.G.S. 513) in the zone of Prionocychis 

 wyomingensis. U.S.N.M. 132285. 



