108 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull.106. 



Genus CALLISTA Poli. 



Gallista (Dosiniopsis?) ORBICULATA PI. & M. 



Pl. xxiv, Figs. 9 and 10. 



Cytherca orbicnlaia Hall and Mock, 1856, Mom. Am. Acid. Arts and Sei. n. s., vol V, 



p. 382, PL 1, Fig. 7. 

 Meretrix orbiculata Meek and Hay den, 1860, Proc. Acad. Nat. Sei. Phila., p. 185. 

 Dione orbiculata Meek, 1864, Smithsonian Check List Invert. Foss. of N. A., p. 13. 

 Callista (Dosiniopsis) orbiculata Meek, 1876, U. S. Geol. Sur. Terr., vol. ix, p. 186, PI. 5, 



Figs. 2 a-c. 



Revised description : 



"Shell thick, suborbicular; Leak moderately elevated and near the 

 anterior side; posterior margin regularly rounded; surface marked by 

 fine, equal, concentric lines. 



u Length, 1.08 inches; height, 1 inch; width, 0.60 inch. 



"Our specimens of this species are merely internal casts, with por- 

 tions of the shell attached. Like that first figured by Prof. Hall and 

 the writer, they present no characters by which we can determine 

 definitely its generic relations. Some of the specimens show the mus- 

 cular impressions to be shallow and ovate in form; the posterior im- 

 pression being, as usual, broader than the anterior. The pallial line is 

 provided with a triangular sinus, which extends a little obliquely 

 upward and forward, about two-thirds of the distance from its base 

 toward the middle of the valves, its sides converging at an angle of 

 about 125° to 130°. 



u Although some of our specimens present exactly the form of the 

 figure above cited, the majority of them differ considerably, being more 

 nearly transversely- ovate in outline, in consequence of the posterior 

 side being more extended and less broadly rounded. These may belong 

 to a different species; but as they vary in this respect, and their mus- 

 cular and pallial impressions, as well as the thickness of the shell, are 

 exactly as in the more orbicular individuals, I do not think it advisable 

 to separate them without being satisfied, from the comparison of better 

 specimens, that they are distinct. 



" Should these more nearly ovate specimens be identical with C. or- 

 biculata, as believed, the specimen first figured in the memoirs of the 

 American Academy of Arts and Sciences must give an incorrect idea 

 of the general form of this species, since a majority of the specimens 

 are much less broadly rounded posteriorly. 



"Some varieties of this species, especially the broader forms, resemble 

 somewhat the species C. Owenana, but they are usually less concave in 

 outline just in advance of the beaks, and the pallial sinus is always 

 different in form, as may be seen by the figures. Some of the varieties 

 also resemble our C. Deweyi in form, as well as in the pallial sinus and 

 muscular impressions, but it is evidently a much thicker shell, and often 

 distinctly more gibbous. 



