CRETACEOUS FOSSILS. 449 



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

 what closely related to Heteroceras f angulatum M. & H. (Palentology of the 

 United States Geological Survey of the Territories, p 484, PL 21, Fig 3), 

 more especially as their species is stated to be " dextral (?) ", and their fig- 

 ures, 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 costae, to belong to a sinistral shell, as they incline in 

 an opposite direction to those of a dextral shell. Mr. Meek states, loc. cit, 

 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 Heteroceras. 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 rela- 

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

 or widely disconnected, becoming more compact with increased growth; 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 considered 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, Black Hills. 



Genus HETEROCERAS D'Orb. 

 HETEROCERAS NEWTONT. 



Plate 15, figs. 1-4. 



Heteroceras Neivtonl Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 40. 



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 spire, below which 



point the shell is deflected to an unknown extent; transverse section of the 



shell circular, or nearly so ; umbilicus very wide in the upper part of the 

 29 B H 



