39 



by an elevated knife-edge carina, giving a deeply channeled convolute 

 character to the spire, the center scarcely rising above the level of the 

 outer carination. Body of the outer volution very slightly rounded near 

 the margin of the flattened spire, and distinctly so to the base below. 

 Aperture as long as the shell, narrow above and gradually widening and 

 rounded at the base. Columella curved and apparently reflected upon 

 the preceding volution. Axis imperforate. 



Surface of the shell marked by very fine, revolving, impressed lines, 

 strongest near the top of the volution and on the lower third ; also by 

 finer transverse striae of growth, parallel to the margin of the aperture, 

 and indicating by their direction that the central portion 6f the outer 

 lip is a little in advance of the top and base. 



Formation and locality. — In limestone, near the top of the Fort Pierr6 

 group. On the Cheyenne Eiver, near French Cre^k, Black Hills, Dakota. 



CEPHALOPODA. 

 Genus HELICOCERAS, D'Orb. 



HeLICOCEKAS STEVENSON!, W. Sp. 

 Plate 14, figs. 4-7. 



Shell large and robust, with an elevated, moderately tapering spire^ 

 which is composed of strong, cylindrical, disconnected, dextrally coiled, 

 volutions, sharply enrolled and leaving an umbilical opening equal to, 

 or a little greater than, one-half the diameter of the volution encircling 

 it. The volutions are at first widely disconnected and rapidly descend- 

 ing, but become more closely coiled below, and in the lower part of the 

 example figured are but slightly disconnected. 



Surface of the shell marked by strong, rounded, annulating costal 

 passing entirely around the shell, and separated by wider concave inter- 

 spaces. On the upper surface of the volution the costa are strongly 

 directed backward, but are recurved, and on the outer face are directed 

 gently forward below and within the umbilicus. The costa are further 

 ornamented by two irregular rows of strong, obliquely-rounded, tuber- 

 culose nodes, the upper range being situated along the middle of the 

 volution, and the other half-way between it and the center of the basal 

 surface The nodes of the two ranges, although usually placed on the 

 same costa, are not constantly so, bat frequently alternate, and m very 

 many cases two of the costa unite at the lower node, coutmuing only 

 as one on the lower side of the volution. 



Septa rather distantly arranged, leaving considerable space between 

 them, the convolutions of a single septum "^''"Py;"^ %'P";=f ^^^^l J^ 

 peripherv of the shell, equal to from two-thirds to three-fourths of the 

 diameter of the volution, at the point examined. Siphonal lobe longer 

 thtn wTde and deeply divided at the extremity into two prmcipal 

 Iranches each of /hich are again deeply divided, and the divisions 



