JUBASSIC FOSSILS. 381 



which is an exceptional feature. Lines of growth apparently coincident 

 with the ridges or nearly so. 



We at first supposed this to be only a strong variety of A. cordiformis 

 M. & H., but on further examination are strongly inclined to the belief that 

 it is a distinct species ; but not having sufficient material to satisfactorily 

 determine the question, we have left it under a varietal name for the pres- 

 ent. Should, however, our suspicions prove correct, on the examination of 

 additional material, it may need a distinct specific designation. 



Formation and locality. — Found associated in the same shaly limestones 

 of Jurassic age, with A. cordiformis, and other Jurassic forms, on the tops of 

 the highest hills, two miles south of the Belle Fourche River, near Bear 

 Lodge Butte, Black Hills. 



DIBRANCHIATA. 



BELEMNITID^]. 



Genus BELEMNITES Agricola. 

 BELEMNITES DENSUS. 



Plate 6, figs. 15-19. 



Belemnites densus M. & H., Proc. Acad. .Nat. Sci. Phil. March, 1858, p. 58 ; ibid. Oct. 1860, 



p. 418. 

 Belemnites densus M. & H., Pal. Up. Missouri, p. 126, PI. iv, fig. 10, and PL v, fig. 1. 



Calcified guard or shell attaining a large size, frequently nearly four 

 inches long below the phragmacone, and often but little less than an inch 

 in diameter at the thickest part ; subcylindrical, ovate, or oval in section, 

 and in the larger individuals sometimes obscurely subquadrangular ; often 

 somewhat laterally compressed ; upper parts straight or sometimes slightly 

 constricted, and more or less rapidly tapering below to an obtuse point, 

 with a narrow, obscure, or faintly-marked groove along one side for some 

 distance above the point, and in some cases extending almost to the lower 

 end of the cavity occupied by the phragmacone. Phragmacone short (?), 

 somewhat rapidly expanding from below upward, the sides diverging at an 

 angle of a little more than 20° and slightly curved, the convex side being 



