88 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



< 1 1 < ii e ! ; r : a (Idoiicarcii) Ifebrascensis, Oweu. 



Plate 29, figs. 5, a, b. 

 Cucullcea Nebrasceniis, Oweu (1852), Report Wiscou., Iowa, ami Mia., 582, pi. 8, figs. 1 aud la. 



Shell rhombic-oval, gibbous, oblique, very thick and strong; pallial border 

 smooth; anterior side forming more or less nearly a right angle with the hinge 

 above, retreating obliquely, with a gentle curve below; base ascending from 

 behind obliquely forward, with a long curve; posterior side truncated at an 

 angle of about 70° with the hinge, slightly concave in outline above, and 

 abruptly rounding into the base below; ligament-area rather large, about two- 

 thirds the entire length of the shell, marked by fine, straight, longitudinal strite, 

 and provided in each valve with eight to ten strong divaricating grooves, which 

 are deflected outward under the beaks, so as to cross the fine longitudinal 

 striae obliquely; beaks elevated, gibbous, incurved at right angles to the 

 hinge, rather distant, and located a little in advance of the middle; posterior 

 umbonal slopes prominently rounded. Surface marked by strong concentric 

 lines, which sometimes form, on large shells, small wrinkles or ridges. 



Length of a medium-sized specimen, 2 inches; height from base to top 

 of umbones, 1.78 inches; convexity of the two valves, 1.75 inches. 



Young individuals are probably provided with obscure radiating coslse, 

 though no traces of them are seen on any of the specimens examined. The 

 hinge is straight, and forms with the longest axis of the shell an angle of about 

 30°. The middle teeth are small and rather irregular, while the lateral 

 elongate teeth, of which there are about three at either extremity of the hinge 

 in each valve, are transversely corrugated, and slope outward below the hori- 

 zon of the hinge-line. The muscular impressions appear to be narrow and 

 not very deep, though the ridge bounding the posterior one is quite prominent. 



Although internal casts aud specimens having the basal margins of the 

 valves a little worn, seem to be exactly equivalve, when well preserved indi- 

 viduals are examined, it will lie seen that the basal bonier of the left valve 

 projects slightly beyond that of the other, and is provided with a shallow 

 groove for the reception of the margin of the same. When the two valves 

 are closed, and viewed on the under side, it will also be observed that the 

 borders are a little warped laterally near the middle. 



