410 GEOLOGY OF THE BLACK HILLS. 



Shell of" medium size or smaller, transversely-ovate, largest on the 

 anterior side of the middle and opposite the beaks. Valves depressed-con- 

 vex, being strongly lenticular in a transverse section when united. Beaks 

 small, ap})ressed, and inconspicuous, situated considerably anterior to the 

 middle of the lenoth ; cardinal line slightlv inclined from the beaks to the 

 extremities, the two sides forming with each other an angle of about 140°; 

 anterior end of the shell broadly rounded below the extremity of the cardi- 

 nal line ; })osterior extremity more narrowly rounded and prolonged; basal 

 linti irregularly arcuate, being somewhat straightened behind the middle; 

 })Osterior umbonal ridge distinct, the cardinal slope narrow and flattened or 

 .slightly impressed; escutcheon large, distinct, and rather deeply impressed; 

 ligament of moderate size. Surface strongly marked by concentric lines 

 of growth. 



In the interior, the muscular impressions, as seen on an internal cast 

 of small size, are well marked, the anterior one large and semi-lunate, the 

 posterior one only about two-thirds as large and subcircular in form. 



Formation and locality. — In Cretaceous strata. No. 4, on Cheyenne 

 River, near Rapid Creek, Black Hills. 



LUCINA VENTRICOSA. 



Plate 11, figs. 14-16. 



Lucina ventricosa H. & M., Mem. Acad. Arts and Sei., voL viii, p. 427. 

 lAicina occidentalis var. ventricosa (H. & M.) Meek, Pal. U. S. Geol. Surv. Terr., p. 

 135, PI. 17, fig. 3. 



Shell of medium size, subquadrangularly-ovate, longer than high, a 

 little widest anteriorly, narrower and truncate behind. Valves convex, 

 lenticular in a transverse section when united; beaks small, appressed luul 

 inconspicuous, but little projecting above the general contour of the shell; 

 anterior end rounded ; basal line forming a broader curve ; posterior truncate; 

 imibonal ridge indistinct and the cardinal slope distinctly flattened, escutch- 

 eon deep, the margin sharply angular, the ligament occupying about two- 

 thirds of its length; lunular area slightly elevated. Surface marked by 

 fine, irregular, concentric lines of grow^th; muscular imprints large and 

 deeply marked. 



