404 GEOLOGY OF THE BLACK HILLS. 



ovate figure, tlie widest part varying from in front of, to behind tlie middle 

 of the lenj^th in different individuals. 



Surface of the valves usually marked by very distinct concentric 

 undulations, though sometimes they are only moderately developed; 

 obli(pie internal rib strongly marked, narrow and pointed near the beak, 

 and gradually widening and deepening posteriorly, the concentric furrows 

 distinctly crossing, and leaving, on the deeply concave channel of the casts, 

 corresponding depressions. Inner layers of the shell highl}- nacreous; 

 outer prismatic layer very thin. 



This species resembles Inoceramus harahini, as it is recognized in the 

 same region, in the general form and in the subcuneate anterior end of the 

 shell; but the transverse furrows are generally more distinctly marked and 

 regular. It is readily distinguished by the oblique sulcus and by the 

 opposite valve being largest, if the example above referred to, and figured 

 on the plate, is rightly interpreted. 



We have great doubts of the identity of the western forms wdth I. 

 harahini Morton, from Alabama, and are inclined to believe they would be 

 more naturall}' classed under a distinct name, as formerly done by Messrs. 

 Meek and Hayden. 



Formation and locality. — In limestone of the Fort Pien-^ Group, at Old 

 Woman Fork of the Cheyenne Eiver, Black Hills, where they occur very 

 abundantly, densely packed together in the rock, and possessing the specific 

 characters and the oblique sulcus in a remarkable constant degree. 



ENDOCOSTEA SULCATA. 



Plate 10, fig. 6. 



f Inoceramus cripsii var. stdcatus EcEmer, Kreid. von Texas, p. 56, PI. vii, fig. 2. 

 ? Inoceramus cri2mi Ecemer, Texas, p. 100. 

 Kot Inoceramus sulcatus Parkiusou. 



Compare Inoceramus convexa H. & M., Mem. Am. Acad. Sci. and Arts, vol. 5, p. 386, 

 PI. 2, fig. 2. 



Shell moderately large, transversely-ovate, with ventricose valves, 

 most gibbous on the anterior central region, becoming more compressed 

 behind and toward the posterior dorsal angle; beaks strong, tumid, situated 

 near the anterior end, slightly enrolled and moderately projecting above 



