302 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



from Solarium by its distinctly pearlaceous shell; and that it will include a 



portion of the Cretaceous species generally referred to Solarium, such as 



S. asterianum, S. Neocomiense, &c, d'Orhigny, as well as the California 



species described by Mr. Gabb, and various others. 



So far as yet certainly known, this genus is confined to the Cretaceous 



system ; but it will probably be found in the Jurassic, and perhaps Tertiary 



rocks. 



R3 :s rg :i ri I <■ I i :a flcxistriala, E. & S. 



Plato 19, tigs. 11, «, b, c, d. 



Solarium flcxislr latum, Evans and Shumard (1854), Proceed. Acad. Nat. Sci. Philad., VI, 163. 

 Margaritella flexistrala, Meok and lluydcii (1880), ib., XII, 423. — Meek (18ii-l), Smithsouiau C'beek-List N. 

 Am. Cret. Fossils, 18. 



Shell small, depressed-convex, terminating externally in a sharp cutting- 

 edge ; volutions four or five ; spire very slightly elevated ; inner edge of volu- 

 tions bounded by a row; of tubercles;* surface marked by fine, revolving, 

 thread-like striae, which are crossed by fine flexuous stria?, giving an exceed- 

 ingly neat reticulate appearance to the shell; umbilicus large; mouth sub- 

 quadrangular. — Evans. and Shumard. 



Breadth of a small specimen, 0.21 inch; height, 0.08 inch. 



This little shell is much depressed or subdiscoidal in form, and composed 

 of five volutions, which are rather acutely angular at the periphery, and ver- 

 tically flattened on the side forming the walls of the umbilicus, around the 

 margins of which they are subangular. On the upper side, they are com- 

 pressed, with an outward slope, though not quite flattened, and more convex 

 below. The umbilicus is very nearly as broad as the widest part of the last 

 whorl, rather deep, and shows nearly or quite all the volutions to the apex 

 of the spire. Around the moderately-distinct suture, on the upper side of 

 the shell, the lines of growth are gathered into a series of very small, regu- 

 larly-arranged wrinkles. The aperture is transversely rhombic-subquadran- 

 gular, and the inner layers of the shell brilliantly pearly. 



Our measured specimens are smaller than those described by Evans and 

 Shumard, which were more- than 0.26 inch in breadth, but seem to agree 

 well in all other known characters. 



Locality and position. — Cheyenne River, near the mouth of Sage Creek; 

 in the upper part of the Fort Pierre group. 



* The words "row of tubercles" are not correctly used here, as I know from drawings of the type- 

 specimen sent to me by Dr. Sbumard, as well as from our own specimens. There are little wrinkles around 

 the inner margin of the volutions, near the suture, that may, perhaps, in some cases assume somewhat 

 the appearance of little tubercles. 



