402 GEOLOGY OF THE BLACK HILLS. 



oped to the same degree, and often it may not be developed until an 

 advanced stage of growth, as in the case with the ridges bordering the mus- 

 cular impressions of Cucutttea and allied forms; but this is no reason for con- 

 sidering it as an abnormal or accidental feature where it does exist, for in 

 many cases its absence may have resulted from partial decay before final 

 imbedding. 



This feature has long been known to exist in shells of this group, but 

 has generally been considered as of accidental occurrence and of no specific 

 value. Dr. F. Rcemer, in his work on the Cretaceous formation of Texas, 

 figures a form possessing it, which he regards as belonging to /. cripsii 

 Mantell, and also refers others to the same, apparently regarding it as only 

 an abnormal feature and not of specific value. Still in his description of 

 the figure on Plate VII, he applies to the shell the varietal name sulcatus* 

 descriptive of this very feature. Mr. Meek also mentions its existence on 

 specimens which he refers to /. cripsii var. barabhu, but which are probabh r 

 distinct from that form and identical with one of those above referred to 

 Some authors, however, have considered it of more importance. Mons. 

 D'Orbigny founded his I. impressus f on the existence of this feature in his 

 types, and Dr. Morton's I. dlveatus is another example. 



Considering the development of this feature to so great an extent, and 

 in several species, not only in America but also in Europe and elsewhere, 

 we deem it other than an abnormal or accidental one, and not onl} T of 

 specific importance, but as marking a distinct generic group, and propose 

 for it the generic name Endocostea, under the impression and belief that 

 it marks the line of recession of the larger or posterior muscular scar, as in 

 the forms of Inoceramus on which this scar has been detected it occupies 

 the same relative position. 



Genus ENDOCOSTEA Whitf. 



Genus Endocostea Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 31. 



Shell resembling Inoceeamus, being bivalve, with both sides convex, 

 but more or less unequal, and composed of two layers, an outer vertically 



* Not /. sulcatum Parkinson, from the Gault of England. 



tThis species is referred to the genus Thetis by Woodward, See Woodward's Manual of recent 

 and fossil shells. 



