382 GEOLOGY OF THE BJ.A(^K HILLS. 



opposite to tlie groove or cicatrix of the lower end of the guard. 'I'lie cone 

 is divided transversely by thin, very closely arranged septa, gradually 

 increasing in distance with the increased growth of the cone. The lower 

 half inch of a specimen containing about thirty-four septa, and the up[)er 

 half of this space having only about ten or eleven. The septa are very 

 gently and regularly concave throughout Apex of the cone subcentrally 

 situated in the guard, the point being directed toward the side marked 

 by the impressed groove or cicatrix, but above it is sometimes reversely 

 excentric. 



Substance of the guard thick and dense, with a very distinct prismatic 

 structure radiating from the line of the constantly receding apex of the 

 phragmacone, and in a transverse section often shows the successive con- 

 centric layers of which it is composed. Surface smooth. 



The species is quite common in some of the Jurassic beds of the Black 

 Hills region, and appears to be very variable in the proportional thickness 

 of the guard, which is the only part found. In the smaller individuals it is 

 usually much more slender than in the older ones, but as the increase in 

 this direction is by concentric additions to the exterior surface, they may 

 continue to grow in this direction long after they have attained their maxi- 

 mum length. In the younger individuals the alveolar cavity is also more 

 nearly central, and the eccentricity is added afterward, as may be seen by 

 the weathered transverse sections showing the greater thickness of the con- 

 centric rings on one side of the cavity. 



Formation and locality/. — In Jurassic rocks on Beaver Creek, and at the 

 top of the gray shales of the same formation, resting on the red marls of the 

 Trias, east of the Belle Fourche, near Bear Lodge Butte, Black Hills. 



