CRETACEOUS FOSSILS. 383 



SECTION IV. 



FOSSILS FROM THE CRETACEOUS ROCKS. 



LAMELLIBRANCHIATA. 



MONOMYARIA. 



PECTIN ID^. 

 Genus SYNCYCLONEMA Meek. 



SYNCYCLONEMA RIGIDA. 



Plate 7, fig. 1. 



Pecten rigida H. & M., Mem. Am. Ac. Sci. and Arts, vol. v, uew ser., PI. 2, fig. 4. 

 Pecten hallii Gabb., Cat. Cret. Foss., 158. 



Syncyclonema rigida (H. & M.) Meek, Smitbsouiau Cbeck List Invert. Foss., p. 7. 

 Syncyclonema rigida (H. & M.) Meek, Pal. U. S. Geol. Surv. Terr., p. 27, PI. IG, fig. 5. 



Shell small, depressed-convex, erect or nearly so, truncate-ovate in 

 outline, widest below the middle, and the height slightly exceeding the 

 width ; hinge line straight, nearly as long or about half as long as the 

 greatest width of the shell below ; auriculations distinct, the anterior one 

 largest, broadly rounded on the lateral margin, and separated from the 

 body of the shell by a very shallow notch only on the margin of the shell ; 

 posterior wing, small, obtusely angnlar, and not at all distinct from the 

 body ; sides of the right valve straight above the point of greatest width, 

 and forming an angle of nearly 90° with each other ; base regularly 

 rounded from the points of greatest width, where the margin is subangnlar. 



Surface concentrically striate, with distinct even strise, as preserved 

 on the cast ; hinge with a very distinct, triangular, concave cartilage pit in 

 the center of the right valve ; muscular impression, if the markings which 

 we see are rightly interpreted, situated on the posterior side of the middle, 

 and nearly apical, and extending along the posterior side just within the 

 thickened border of the valve to near the middle of the height. Surface 

 and substance of the shell unknown. 



