84 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



lateral or moderately inequilateral ; margins nearly or quite closed, and smooth 

 or crenate within; beaks ventricose, incurved, and generally remote; surface 

 with radiating cost* or striae, which usually differ somewhat on the opposite 

 valves, and sometimes become nearly obsolete; concentric striae more or less 

 distinct; epidermis scaly; hinge straight, of variable thickness, provided with 

 short, vertical denticles along the middle, and longer transverse or declining 

 cross-striated linear teeth, in front and behind; cardinal area well defined, 

 usually broad, and marked by distinct divaricating furrows ; muscular and 

 pallial impressions distinct ; posterior muscular scars each connected with a 

 prominent, oblique plate or lamina.- 



This genus is divisible into three subgenera, as follows : 



1. cucull-sja, Lamarck (typical). 



Shell comparatively thin ; hinge-margin very narrow, with most 

 of its length occupied by the small vertical denticles; lateral denticles 

 few, straight, horizontal and short; laminae of posterior muscular 

 scars very prominent. — (Type as stated above.) 



2. idonearca, Conrad. 



Shell thicker than the last, with a wider hinge-plate and fewer 

 of the short mesial denticles, which are prominent and transversely 

 striated ; lateral denticles long, horizontal, or descending outward, and 

 bent downward at the inner ends, strongly cross-striated ; laminae of 

 posterior muscular scars prominent. — Cucullcea Tippana, Conrad. 



3. latiarca, Conrad. 



Shell very thick and strong; hinge comparatively narrow, and 



occupied by very irregular, strong, vertical denticles, excepting at each 



end, where it is broad, and occupied by strong, rugosely-striated, longer 



denticles, declining outward, and bent downward at the inner ends ; 



posterior muscular scars, with their lower margins somewhat raised and 



acute, but not forming a projecting lamina. — Cucullcea onoclca, H. I). 



and Wm. 13. Rogers. 



The genus Cucullcea, including the above sections, and possibly some 



others not yet defined, ranges back to the Lias, and continues through the 



subsequent formations; probably attaining its greatest development in the 



Cretaceous. Most of the Cretaceous species seem to fall into the Idonearca 



and Latiarca groups, though possibly some of them will form distinct sections. 



The Tertiary species mainly belong to Latiarca and the typical sections, 



