90 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



ular, rough sulcus (possibly due to accident) extending from near the beak 

 of each valve obliquely backward nearly to the base of the shell, a little 

 behind the middle. None of the few specimens, yet seen, show the hinge 

 or other internal characters. 



This species differs from the last in being higher in proportion to its 

 length, and in having more elevated beaks, while the anterior side, hinge, 

 and ligament-area, are much shorter. It seems not to be very nearly related 

 to any foreign species with which I am acquainted. 



Locality and position. — Moreau River and Long Lake ; Fox Hills group, 

 or No 5 of the Cretaceous series of the Upper Missouri country. 



Genus TRIGONARCA, Conrad. 



Synon.—Trigonarca, Courad (1862), Proceed. Acad. Nat. Sci. Philad., XIV, 289; and (1867), Am. Jour. 



Couch., Ill, 9; also (1872), Proceed. Acad. N. Sci. Philad., XX, 54; and (1873), in 



Appendix to Kerr's Report, Geol. Survey of N. Carolina, 2. — Stoliczka (1871), Palseout., 



Iudica III, number V, 337. 

 Breviarca, Courad (1872), Proceed. Acad. Nat. Sci. Philad., XX, 54 ; and (1873), Appeudix to Kerr's 



Geological Report N. Car., 3 (as a subgenus of Trii/onarca). 

 Cucullwa, Pectunculus (= .lxinaa), and Area (sp.), of several authors. 



Eh/m — rplyavoc, triangular.; and Area. 

 Type — Cucullcea Alaeonensis, Conrad. 



Shell thick, gibbous, equivalve, varying from subtrigonal to subtrape- 

 zoidal, orbicular or suboval ; hinge-area small or moderately developed, with 

 cartilage- furrows longitudinal, and divaricating between the beaks, or minute 

 and transverse; hinge more or less arched, narrow in the middle, and often 

 widening toward the extremities, provided with numerous radiately-arranged, 

 generally straight denticles, the lateral of which are longer than the others; 

 posterior muscular scar provided with a slightly-projecting marginal lamina, 

 somewhat as in Cucullcea; surface marked with concentric striae, and often 

 obscure, or obsolescent radiating strife or costa?.. 



The above synonymy and diagnosis include two rather distinctly- 

 marked groups, or subgenera, distinguished as stated below : 



1. trigonarca, Conrad (typical). 



Shell subtrigonal or subtrapczoidal, with posterior margin 

 obliquely truncated, and posterior basal extremity more or less 

 angular; posterior umbonal slopes prominently rounded or angular; 

 hinge-area divaricately furrowed; hinge-teeth rather strong. — (Type 

 as stated above.) 



