304 (JEOLOGY or THE BLACK HILLS. 



Sliell large, tlie f^eneral form being oblate or more or less transversely 

 broad-oval, with inoderatel}' convex valves, and large, somewhat tumid, and 

 slightly incurved beaks, situated but a short distance from the anterior end 

 and projecting above the line of the hinge. Hinge line st-aight, equal to 

 about half or a little more than half the length of the shell below; anterior 

 and posterior ends rounded, the former longest above the middle and the 

 latter below the middle of the valve; basal line obliquely rounded, more 

 or less prolonged behind in the direction of the umbonal ridge; extremity 

 of the hinge line rounding to the posterior margin, with a long and gradu- 

 ally increasing slope. Valves most ventricose along the umbonal ridge and 

 on the umbones, retaining considerable ventricosity toward the anterior 

 mai-gin, which is often quite abrupt, but sloping more gradually posteri- 

 orly, and becoming much compressed and flattened on the postero-cardinal 

 extension. 



Surface of the valves marked by strong concentric undulations of very 

 varying size and distance, and often by finer concentric lining in the depres- 

 sions between. Faint indications of radiating lines are also frequently seen 

 on the exfoliated surfaces. 



The specimens of this species are subject to very great variation; in 

 fact, there is scarcely a single feature,that is in any degree constant in its 

 character through many individuals. ■ In convexity they var}- from depressed 

 convex to highly gibbous. They also differ exceedingly in the strenglh 

 and number of the concentric undulations, some having been noticed wdiere 

 the undulations become obsolete and the shell suddenly geniculated, giving 

 a stronolv deformed character to the valves The extreme variation of the 

 species has been noticed by others; and Mr. ]\Ieek remarks, under his 

 description of the form on page 53, Paleontology of the United States 

 Geological Survey of the Territories, when speaking of Dr. ( )wen's species, 

 that "a careful study of our specimens, in connection with Dr. Owen's 

 figures and description of his 1. sagensis and I. nebrascensis, has led to 

 the conclusion that the forms for which these names were proposed are 

 not specifically distinct"; and in the following paragraph he further says, 

 in speaking of the variation of the outline between his and Dr. Owen's 

 figures of T. )iehra.sreiisis : "There a])pears, howevei", to Ijo so many gra- 



