INVERTEBRATE PALAEONTOLOGY. 325 



A 11 « liiir a (D re pa ii 6 c hi I us) Americana, E. & S. (sp.). 



Plate 32, tigs. ,-, a, b. 



Roslellaria Anurkana, Evans anil Slininai'd (1857), Traus. St. Louis Acad. Sci., I, 42 (not d'Orbifjuy, 182G). 

 Aporrhais Americana, Meek and Haydcn (I860), Proceed. Acad. Nat. Sci. Pliilad., XII, 423. 

 Anehura (Drepanochilus) Americana, Meek (1864), Smithsonian Check-List N. Aui. Cret. Fossils, 19. 



Shell rather small, exclusive of the lip unequally fusiform ; spire rather 

 acutely conical, and about equaling the length of the aperture and canal ; 

 volutions seven and a half to eight, moderately convex, last one unequally 

 biangular around the middle, obliquely flattened above, and abruptly con- 

 tracted into the rather short, straight, mucronate beak below; upper carina 

 or angle of the body-volution more prominent than the other, usually subcre- 

 natc before reaching the lip, on which it is strongly defined to its upward- 

 curved extremity; suture well defined; outer lip of the breadth of the 

 body-volution at its commencement, but rapidly narrowing outward, and 

 curving strongly upward to its acutely-pointed extremity, almost without a 

 lower marginal sinuosity, but having a broad, rounded sinus above; inner lip 

 rather thick all the way up, and connecting above with outer; aperture 

 rhombic-oval and somewhat oblique ; surface ornamented by numerous, very 

 regular, strongly-arched, little, vertical costae, and very fine, regular, thread-like, 

 revolving lines; costae generally obsolescent on the body-volution, where the 

 revolving lines become somewhat stronger, below the lower carina, which is 

 itself continued only as a stronger line on the lip, without producing the 

 slightest angularity to the outline of the latter. 



Length, 1.08 inches; breadth of body-whorl, 0.38 inch; slopes of spire 

 a little convex, with a divergence from the apex of about 30°. 



This is one of the most abundant and beautiful of the Upper Missouri 

 Cretaceous univalves, and is often found so finely preserved as to show nearly 

 all of its characters as clearly as we usually see in recent shells. Its lines 

 of growth are very obscure on the upper volutions, but become more distinct 

 on the last one, and on the expanded lip. The little vertical costse are quite 

 well defined, and very regular on the spire, but become much more obscure, 

 or even only appear as little cren illations, on the larger carina of the body- 

 volution. The revolving lines are exceedingly fine, regular, and crowded on 

 the volutions of the spire and the upper slope of the body-whorl, but become 

 stronger on the lower side of the same, and as they approach the margin 



