INVERTEBRATE PALEONTOLOGY. 289 



The six radiating hair-lines arc generally sharply impressed, but where 

 the surface is a little worn tiny sometimes become nearly obsolete. They are 

 never represented by any corresponding lines, furrows, or ridges on the 



interior of the shell, so far as can l>e determined from internal casts. The 

 general obliquity of the summit is toward the Trout : hut the very small. 

 pointed, immediate apex, is suddenly thrown or folded backward, though it 

 is very rarely preserved entire as the specimens are usually seen. As seen 

 broken or worn oil", the apex sometimes looks very much as if it were per- 

 forated by a minute opening; but, as already stated, this is probably merely 

 accidental. 



None of the internal casts yet examined show the whole muscular 

 impressiou, but some of them show it to become abruptly attenuated as it 

 approaches the point on the right posterior side, where the interruption is 

 seen in that of the other species. 



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

 from the base of the Fort Pierre group. 



Aniso m y o ii S h n mardi. M. & H. 



Plate 18, figs. 7. <i, h. .. 

 Anismiyon Shumardi, Meek and Harden | 1860), Proi 1. Acad. Nat. Sci. I'liilad., XII. 423. 



Shell obliquely conical, a little compressed laterally: aperture oval or 

 subcircular; summit elevated, located a little in advance of the middle; 

 immediate apex apparently presenting the usual pointed and recurved char- 

 acter: lateral slopes concave in outline below, nearly straight above: anterior 

 slope slightly convex above, and concave below: posterior side nearly straight 

 or a little convex, and provided with a rather obscure, narrow, mesial carena, 

 which extends'from the apex nearly to the base; surface marked by small, 

 inconspicuous lines of growth, which are crossed by obscure traces of fine, 

 radiating strife, and six equidistant, impressed hair-lines, two of which extend 

 from the apex to the anterior border, and four from the apex to the posterior 

 and posteror-Iateral margins. 



Length of base, about 1.4b" inches: breadth of base, about 1.32 inches. 



This form resembles the last in its general appearance, and has the 

 same surface-characters, but differs in being more elevated anil in having its 

 apex much more nearly central, with the abrupt anterior slope convex instead 

 37 h 



