CRETACEOUS FOSSILS. 143 



ally longer and more depressed than any otherwise nearly allied species with 

 which I am acquainted. 



Locality and position. — North Platte River, above Platte Bridge, in Da- 

 kota Territory; from the Cretaceous formation No. 2 or 3 of the Upper Mis- 

 souri section. Discovered by Colonel Simpson. Museum of the Smith- 

 sonian Institution. 



iNOCERAMUS PROBLEMATICUS, Scblot.?. ° 



Plate 13, figs. 2 and 2 a. 



Mytilites problematicus, Scblotb. (1820), Petref., 312. 



Inoccramus mytiloides, Sowerby (1823), Min. Conch., V, 61, pi. 442; Goldf. (1836), 



Petref., II, 118, tab. cxiii, fig. 4. 

 Caiillus Schlotheimii, Neilsson (1827), Petref. Suecaua, 19. 

 Catillus mytiloideSf Deshayes (1830), Encyc. Meth., II, pi. 211. 

 Inoceramus prohlematicus, d'Orbigny (1843), Pftleont. Fr., Ill, 510. — Meek (1873), 



Hayden's Sixth Report, 476; and (1876) in Col. Simpson's EeportExpl. across 



Great Basin of Utah, 358, pi. 4, fig. 1 a. 

 Compare I. mytiloidcs, Roemer (1852), Kreid. von Texas, 60, pi. vii, fig. 5 (= J. myti- 



Zopsts, Conrad (1857), U. S. and Mex. Bound. Eeport, 1, 152, pi. 5, figs. 6«, and 6 6; 



also ^ ith 7. pseudo-mytiloides, Schiel (1855), Pacific Railroad Eeports, II, pi. 3, 



fig. 8. 



Shell obliquely subovate, extremely inequilateral, rather compressed, 

 and apparently nearly equi valve ; anterior margin truncated or sloping very 

 obliquely backward fi*om the beaks to near the middle, where it passes im- 

 perceptibly into the base; basal margin sloping obliquely backward and 

 rounding into the posterior basal extremity, which is generally narrowly 

 rounded; hinge-line rather short and very oblique to the longer axis of the 

 valves; posterior dorsal margin sloping obliquely with a more or less convex 

 outline from the posterior extremity of the hinge to the posterior basal mar- 

 gin; beaks very oblique, acutely pointed, incurved, and terminal. Surface 

 ornomenied with small, more or less regular, concentric undulations and 

 stria;. 



At the time I wrote the above description, I had seen only the figured 

 specimens, which are much broken and distorted. Since that time, I have \jy.^ >- '^ 



had an opportunity to collect and examine a large series at the same locality '^ J^*^ lo 

 in Wyoming from which those figured on plate 13 were collected. These ^ ^c^ 



^ditional specimens show that this shell varies greatly in form ; there being 

 apparently an unbroken series from specimens like those figured on our 



