52 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



with the anterior margin of about 1.20° ; anterior side somewhat prominent 

 and regularly rounded; posterior side extended and probably subtruncate. 

 Surface marked by strong undulations, which are simple at the extremities, 

 while some of them become divided toward the center; being less conspicuous 

 toward the beaks, and almost obsolete near the base of the shell. Obscure 

 concentric lines mark the surface of exfoliated specimens somewhat irreg- 

 ularly. ■ 



Although several very imperfect examples apparently of this form occur 

 in the Upper Missouri collections under consideration, none have yet been 

 found in quite so good a condition as the original typical specimen, the figure 

 of which is copied on our pi. 12. From the limited means of comparison 

 at this time available, I am left in doubt in regard to the propriety of retain- 

 ing it as a distinct species. Dr. Zittel cites it, in his work on the Gosau 

 Fossils, as a synonym of I. Cripsii, which may be correct; but we have not 

 yet the means of confirming or positively disproving his opinion. We have, 

 however, as good or perhaps even better reasons for supposing it a variety 

 of /. Sagensis, Owen. On comparison with Owen's figure of the type of 

 /. Sagensis (taking care to place both figures with the hinge in a horizontal 

 posture), it will be seen to differ mainly in its less prominent beaks, more 

 convex anterior outline, and the curve of its undulations indicating a more 

 transverse form ; in nearly all of which characters, however, it agrees more 

 nearly with /. Nebrascensis of Owen. My present impression is that it is 

 probably distinct from both of Owen's types. 



Localit;/ and position. — Great Bend of the Missouri, below Fort Pierre; 

 from the Fort Pierre group of the Upper Missouri Cretaceous. 



| ii o c era in us Sagensis, var. Nebrascensis, Owen. 



Plate 13, figs. 2, a, h. 



Inoceramus Nebraacensis, Owen (1852), Report Min., Iowa, aud Wisconsin, 582, pi, 8, fig. 1. 

 I. Sagensis, Owen (1852), ill., pi. 7, lig. 3. 



Compare /. convexus, Hall and Meek (1854), Mem. Am. Acad. Arts and Soi., V (n. s.), 386, pi. 2, fig. 2; 

 and our figs. 5, a, b, pi, 12. 



Shell large, obliquely broad-ovate or subcircular, moderately gibbous, 

 about as high as long; anterior side short, making a very broad, oblique 

 curve from the beaks to the base; ventral and postero-ventral margins nearly 

 regularly rounded; hinge rather short, forming an angle of about 50° with 



