62 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Subgenus MYTILOIDES, Brong. 



Inoceramas problematicns, Schiot 



Plate 9, figs. 3, a, h. 



Mytilites problematicns, Schiot., Petrefaot., 312. 



Inoeeramiis mytilloides, Mantell (1823), Geol. Sussex, '215, pi. 28, fig. 2. — Sowerby (1823), Min. Conch., V, 



61, pi. 442.— Gol.lf. (1836), Petrefact. Germ., II, 118, pi. cxiii, fig. 4. 

 Iiwceramus prdbUmaHeus, d'Orbigny (1843), Pal. Franc, III, 510, pi. 406. — Meek aurl Ilayden (May, 1857), 



Proceed. Acad. Nat. Sci. Philad., IX, 119. 

 J Inoeeramiis pseudo-mytiloides Scliiel (1855), Pacific Railroad Reports, II, 108, pi. 3, fig. 8. 



Shell obliquely elongate-oval, subelliptical or ovate, nearly or quite equi- 

 valve, rather compressed, thin and fragile ; anterior side forming a slightly 

 convex curve from the beaks obliquely downward and backward ; postero- 

 basal extremity rather narrowly rounded ; postero-dorsal margin very oblique, 

 compressed, nearly straight, or sometimes a little convex in outline below the 

 middle, and slightly concave above ; cardinal border short, straight, com- 

 pressed, and forming an angle of about 45° with the longest diameter of the 

 shell; beaks terminal, rather small, nearly equal, obtusely-pointed, rising 

 little above the hinge, and not much incurved. Surface ornamented by more 

 or less regular, concentric undulations, and smaller marks of growth. 



Greatest length, 4 inches ; breadth, at right angles to the longest diam- 

 eter, about 2 inches ; convexity of the two valves, about 0.80 inch. 



Some specimens of this shell are proportionally a little wider than the 

 figures generally given of I. problematicns, while others seem to lie almost 

 exactly like those published by Goldfuss and d'Orbigny. Like that species, 

 it also varied much in form at different ages, being proportionally broader 

 and more convex when young, than at more advanced stages of its growth. 



Locality and position. — Big Sioux River, six miles above its mouth ; on 

 Little Blue River near Kansas and Nebraska line, and at many other locali- 

 ties in that region ; also at Coalville, Utah, Fort Harker, Montana, as well as 

 in Wyoming, Colorado, Arkansas, and southward into Texas ; and at numerous 

 foreign localities. Occurs in lower part of Niobrara group, or formation No. 3, 

 of the Upper Missouri Cretaceous series, and ranges down into No. 2, or the 

 Fort Benton group of the same. 



