54 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITOKIES. 



the other, and more or less broadly rounded; base semicircular or semi-oval, 

 usually a little more prominent behind than in front of the middle; anterior 

 margin rounded ; hinge straight and comparatively long, ranging nearly 

 parallel to the longer diameter of the shell. Beak very small, compressed, 

 projecting but slightly above the hinge-line, and located about one-fifth the 

 length of the shell back from the anterior margin. Surface ornamented by 

 small, very regular, simple, concentric undulations or cost;e, which are well 

 marked on internal casts. 



Length, about 2. -.0 inches; height, 1.96 inches; convexity (left valve), 

 0.42 inch. 



I have seen but a single specimen of this shell, from the Upper 

 Missouri, and consequently have no means of judging to what extent it may 

 vary. It resembles Inoceramus confertim-annulatus of F. Roemer (Kreid. 

 von Texas) so closely that I have been almost inclined to refer it to that 

 species. It differs, however, from Roemer's figure in the following particu- 

 lars : first, its antero-ventral margin is much less prominent; secondly, its 

 hinge-line is longer ; thirdly, its beaks are placed farther forward, while the 

 height of the valve is greater in proportion to its transverse diameter. If 

 these, difi'erences are constant, we must regard these two forms as specifically 

 distinct; if not, however, unless the other valve may be found to present 

 other difi'erences, it would be difficult to point out any characters by which 

 they can be separated from Roemer's species. 



From /. Bambini of Morton, as figured on pi. 17 of his Synopsis Org. 

 Rem. (not fig. 11 of pi. 13, which may be distinct species), with the original 

 of which we have compared it, this species differs in being more compressed, 

 less oblique, and in having more regularly and more closely-arranged con- 

 centric undulations or costae. 



From all the other species yet known from the Upper Missouri rocks, 

 resembling it in other respects, this shell will be distinguished by its more 

 compressed form and smaller and more regular concentric costae. It is not 

 near so high in proportion to its transverse diameter nor so broadly rounded 

 posteriorly as I. Vanuxemi, which resembles it in the character of its con- 

 centric costae as well as in the slight convexity of the valves. 



Since proposing to name this species after Dr. Morton, under the 

 •impression that it had not previously been described, I have seen specimens 

 from Eufaula, Ala., regarded as authentic examples of Professor Tuomey's 



