white.] PALEONTOLOGY — CEETACEOUS FOSSILS. 289 



Genus CYKENA Lamarck. 



Cyeena securis Meek. 



Plate 3, figs. 2 a, t, and c. 



CorUeula {Cyrena f) securis Meek, 1873, An. Eep. U. S. Geol. Surv. Terr, for 1872, p. 494. 

 Cyrena (Ydoritina) erecta "White, 1876, Powell's Rep. Geol. Uinta Mts., p. 117. 



Shell of medium size, obliquely subovate in marginal outline when 

 adult, but subcircular when young, gibbous, especially the upper median 

 portion, but somewhat laterally compressed at the postero-basal por- 

 tion ; antero-basal, basal, and postero-basal borders forming a continuous, 

 almost regular, curve ; the dorsal outline, by lateral view, also broadly 

 rounded from the beaks to the postero-basal border; front a little con- 

 cave transversely below the beaks, and also vertically concave from the 

 beaks to a point a little below the inidheight of the shell ; dorsum lon- 

 gitudinally concave from between the beaks to a point on the posterior 

 margin a little below the midheight of the shell; beaks prominent, 

 approximate, and curved forward; ligament short and narrow. This 

 concavity of the dorsum resembles an escutcheon, except that it is not 

 denned, especially at the ends; it is moderately deep, narrow, and 

 bounded at the sides by the abrupt rounding inward and downward of 

 the surface from the outer side of each valve, so that the hinge-margin 

 is wholly hidden from sight by a side view of the shell; hinge and inte- 

 rior unknown. Surface marked by the ordinary hues and imbrications 

 of growth. 



Height, 33 millimeters; antero-posterior width the samej thickness 

 about 22 millimeters. 



Mr. Meek's description was made from a single cast in sandstone 

 obtained from strata that are evidently equivalent with those from which 

 the example figured on plate 3 was obtained ; the localities being upon 

 opposite sides of an anticlinal axis, and only about three miles apart. 

 This species, as pointed out by Mr. Meek, and shown above, has the 

 external characteristics of the forms which he separated as a subgenus 

 of Corbicula under the name of Ydoritina. Perhaps it belongs to that 

 group, but it is more probably related to Cyrena dalcotaensis Meek & 

 Hayden, from the Dakota Group of the Upper Missouri Cretaceous series, 

 and C. inflexa Meek, from a higher Cretaceous horizon near Gallatin 

 City, since it is, like those species, associated with marine forms; 

 while the species of the group Veloritina, so far as they are fully known, 

 are all associated with brackish- and even some fresh-water forms. If 

 this supposition is correct, we have the remarkable fact of certain species 

 belonging to two genera respectively assuming an external form that 

 has been regarded as of subgeneric value. This, perhaps, is not impos- 

 sible, especially in this case, because the generic difference, as shown 

 by the shells, between Cyrena and Corbicula, is neither clear nor satis- 

 factory. Among the numerous examples of shells having the external, 

 and in part the internal characters of Corbicula which have been found 

 associated with marine forms in Cretaceous strata, I have never detected 

 transverse striation upon their lateral teeth. Because of this and other 

 differences from true Corbicula, I refer all these marine Cretaceous forms to 

 Cyrena, but they probably also differ from the typical forms of that genus. 



Position and locality. — Cretaceous strata of the Fox Hills Group; 

 Hilliaid. Station, Union Pacific Eailroad, Wyoming. In my original 

 description of the species, as C. erecta (loc. cit.), it was mentioned as 

 occurring at Upper Kanab, Southern Utah, but those examples are now 

 thought to belong to another species, and to a higher horizon. 

 19 GS 



