40 



furnished with numerous serrated digitations. First lateral lobe very 

 large and unequally divided into two principal branches, the outer one 

 of which is the largest, and each is again divided and furnished with 

 numerous compound digitations. The second lateral lobe is much 

 smaller than the first, and similarly divided, but the divisions are smaller 

 and less complicated. (The inner half of this lobe has not been distinctly 

 traced.) First lateral sinus rather large and deeply cleft in the middle 

 by an auxiliary lobe, the lower end of which is divided into several digi- 

 tations. Second lateral sinus shorter and comparatively more spreading 

 than the first also, and deeply divided in the middle by a simple auxili- 

 ary lobe having four spurs on each side. Other lobes and sinuses not 

 observed. Siphuncle rather small, marginal, and passing along the mid- 

 dle of the volution. 



This species would appear, without critical examination, to be some- 

 what closely related to Heteroceras f angulatum, M. & H. (Pal. U. S. Geol- 

 Surv. Ten-it., p. 484, pi. 21, fig. 3), more especially as their species is 

 stated to be "(dextral?)"; and their figures, being placed base upward, 

 which may be seen by the position of the nodes, are liable to mislead, 

 but if critically examined will be found, from the direction of the costa, 

 to belong to a sinistral shell, as they incline in an opposite direction to 

 those of a dextral shell. Mr. Meek states loc. cite that the fragment 

 figured is supposed to be from the deflected part of the shell, and for 

 this reason is placed provisionally, with doubt, under the genus Hetero- 

 ceras. It is, perhaps, impossible for one to feel positive that a shell of 

 this group has not been, or would not be, deflected at some period of 

 growth, unless a perfect adult individual is obtained. Our shell figured 

 shows no evidence whatever of any tendency to become deflected ; still 

 it is known to be imperfect, as it is septate to within an inch of the 

 outer extremity, and we cannot therefore feel positive of its true generic 

 relations. The upper part of the spire has, however, been very loosely 

 coiled, or widely disconnected, becoming more compact with increased 

 growth 5 in this respect being the reverse of shells belonging to the genus 

 Heteroceras. Owing to this feature, and the absence of any evidence of 

 the generically-required deflection in the adult stages, we have con- 

 sidered it most natural to refer it to Helicoceras. 



Formation and locality. In limestone of the Fort Pierre group, on 

 the west side of Beaver Creek, southwest Black Hills. 



GENUS HETEROCERAS, D'Orb. 

 HETEROCERAS NEWTONI, n. sp. 



Plate 15, figs. 1-4. 



Shell dextral, of moderate size, composed of from two and a half to 

 three, loosely coiled, disconnected, and rather rapidly increasing 

 volutions forming the rather low but somewhat rapidly spreading 



