392 GEOLOGY OF THE BLACK HILLS. 



INOCEEAMTJS PEEPLEXUS. 



Plate 8, fig. 3, and PI. 10, figs. 4, 5. 



Inoeeramus perplexus Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 31. 



Shell rather below the medium size, erect, truncate subovate in outline, 

 shortest at the hinge line and gradually expanding to below the middle of 

 the height ; valves convex ; beaks terminal, slightly inclined forward, 

 pointed and incurved ; hinge line short and straight, not exceeding two- 

 thirds of the length below in any of the examples seen ; anterior border 

 straight or very slightly concave for more than half the height of the shell, 

 when it unites with the deeply rounded basal border ; posterior margin of 

 the shell gradually widening from the extremity of the hinge line to the 

 point of greatest width in an antero-posterior direction, which is rather 

 below the middle of the height where it unites in a regular curve with the 

 basal margin. Valves most strongly convex along the umbonal ridge, 

 which is situated nearest to the anterior side of the shell and slopes gradu- 

 ally to the postero-cardinal portion, where it becomes almost flattened ; 

 while the anterior border is vertical, or sometimes slightly concave, for 

 more than half the depth of the valve. 



Surface of the shell marked by strong, irregular concentric folds or 

 undulations parallel to the margin, and passing off on the vertical anterior 

 border. Substance of the inner layers highly nacreous. Fibrous coating, 

 when preserved, apparently thin. 



This species is somewhat remarkable for its erect form and strong con- 

 centric undulations. In form it resembles the specimens of I. fragilis H. 

 & M., given by the latter author (Paleontology of the United States 

 Geological Survey of the Territory, p. 42, Figs. 1 and 2), but in the surface 

 characters it is entirely distinct from that species, and when compared with 

 examples of that species having the characters of the original specimen is 

 very readily distinguished. It also somewhat resembles I. ellioti Grabb., 

 from the Cretaceous sandstones of San Francisco Harbor, California. 



Formation and locality. — In concretionary limestone at the top of shales 

 of Cretaceous No. 2, on the Belle Fourche, about ten miles west of Crow 

 Peak, Black Hills. 



