626 SYSTEMATIC PALEONTOLOGY 



Description. " Shell small, very thin and pearly, obliquely ovato- 

 cordate, ventricose ; postero-basal and basal margins rounded ; dorsal 

 border sloping posteriorly with an arcuate outline, and rounding into the 

 anal margin behind; anterior border rounding obliquely backwards into 

 the base; umbonal region of both valves very gibbous, beaks prominent, 

 terminal, pointed, distinctly incurved and directed obliquely forward at the 

 extremities; hinge margin smooth ; free border minutely crenulated. Sur- 

 face (as seen by aid of a magnifier) beautifully ornamented by extremely 

 fine, regular, closely-arranged, radiating striae, which increase chiefly by 

 bifurcation, and continue of 'uniform size on all parts of the shell ; cross- 

 ing these are numerous, equally fine, but much less distinct, concentric 

 lines, and occasional stronger marks of growth. Length, measuring 

 obliquely forward and upward from the base to the beaks, 0.55 in. ; diam- 

 eter, from base to hinge, measuring at right angles to the greatest length, 

 0.4 inch; convexity, 0.37 inch. This beautiful little shell is very closely 

 allied to C. sericea of Conrad, but differs in being uniformly more broadly 

 ovate in form, and in having less elevated and less distinctly incurved 

 beaks, while its concentric markings are not near so strongly defined."- 

 Meek and Hayden, 1862. 



Type Locality. Deer Creek, near North Branch of the Platte Eiver, 

 Nebraska. 



The species is recognized in Maryland from casts only. It is more than 

 double the size of C. serica Conrad, relatively broader, and less inflated 

 and less prominently sculptured concentrically. 



Occurrence. MONMOUTII FORMATION. Brightseat, Prince George's 

 County. 



Collections. Maryland Geological Survey, New Jersey Geological Sur- 

 vey, U. S. National Museum. 



Outside Distribution. Monmouth Formation. Tinton beds. New 

 Jersey. Ripley Formation. Exogyra costata zone, ? Owl Creek, Tippah 

 County, Mississippi. Fox Hills Sandstone. Western Interior. 



