MESOZOIC FOSSILS. 635 



by lines of growth and by regular, narrow, elevated, concentric ridges that 

 are about one-third as wide as the interspaces. These ridges are somewhat 

 stronger on the anterior half of the shell than on the posterior, and in very 

 large specimens they tend to become obsolete, making the basal portion of 

 the shell nearly smooth. 



The above description is drawn mainly from a large right valve 

 (PI. LXXVI, fig. 1) from the Sickle Creek locality. The specimens associ- 

 ated with it and having the same general form and sculpture are all much 

 smaller. These are not distinguishable from specimens from sandstone 

 believed to belong to a higher horizon on Glade Creek. There are small 

 left valves in the collection from both localities, and one of those from 

 Glade Creek is figured. It is proportionally more convex than the right 

 valve, and the beak is more prominent and more curved. The concentric 

 ridges are very prominent on the convex median region, and fade out 

 toward the borders. 



The largest type specimen measures 201 mm. in its longest diameter, 

 and 135 mm. at right angles to that line. 



This species is related to /. fragilis and I. altus, all three belonging to 

 the typical section of Inoceramus. It differs from both of them in being 

 more strongly plicate, in its shorter, slightly more oblique hinge line, and in 

 other details of outline. 



Locality and position: On Snake River one-fourth mile above the 

 mouth of. Sickle Creek, associated with I. umbonatus, Scaphites ventricosus, 

 etc., in sandy shales of the upper part of the Colorado formation, and near 

 the mouth of Glade Creek in a sandstone supposed to belong to a higher 

 horizon. 



Corbula subtrigonalis Meek and Hayden. 



Corbula subtrigonalis Meek aud Hayden, 1856: Proc. Acad. Nat. Sci. Phila., p. 116. 

 White, 1880: Ann. Eept. U. S. Geol. Surv. Terr, for 1878, p. 80, PI. XXV, figs. 

 6a-/. White, 1883: Third Ann. Rept. U. S. Geol. Surv., p. 442, PI. XIX, figs. 

 10-13. Stanton, 1894 : Bull. U. S. Geol. Surv. No. 106, p. 123, PI. XXVII, figs. 7 

 and 8. 



This species and its variety perundata were obtained in black shales 

 supposed to belong to the Colorado formation near Electric Peak and on 

 the Cone head of Gardiner River. These forms were originally described 

 from the Laramie, but they are known to range as low as the Colorado 

 formation in southwestern Wyoming. 



