454 GEOLOGY OF THE BLACK HILLS. 



of tripartite; but this character does not appear to hold good among many 

 of those referred to the genus; and, as we have not seen the septa of the 

 type species, we are uncertain if it is an original feature of the genus or 

 only an erroneously added one. 



Formation and locality. — In limestone referred to the Fort Pierre' Group 

 of the Upper Missouri Cretaceous, on the east fork of Beaver Creek, three 

 miles west of Camp Jenney, Black Hills. 



ANCYLOCERAS TRICOSTATUS. 



Plate 15, figs. 7, 8. 

 Ancyloceras tricostatus Whitf., Prelim. Eept. Pal. Black Hills, 1877, p. 43. 



A single fragment of a sinistrally helicoid shell, the volutions of which 

 have been entirely separate and coiled upon the same plane, with a rapidly- 

 increasing curvature, appears to possess features entirely distinct from those 

 of any described species. The form of the volution has been quadrangu- 

 larly ovate in section, vertically compressed, with the greatest vertical thick- 

 ness at about the inner third of the width, and slightly flattened on the 

 dorsum. The shell is marked by a line of strong angular nodes on each 

 dorsal angle, and by closely-arranged subangular encircling ridges or 

 costa, which are directed slightly forward in crossing from the ventral to 

 the dorsal margin on the upper side of the volution, and much more 

 strongly so in crossing from the upper to the lower side of the dorsal flat- 

 tening, and are recurved below and on the umbilical surface. The costse 

 are arranged in sets of three, the two adjacent ones of which unite in the 

 nodes on the upper and lower dorsal angles, while the third ridge of each 

 set encircles the shell between the nodes of two adjacent sets. Intercostal 

 spaces concave. 



Septa very complicated and distant, the spaces between two adjacent 

 ones equal to about one-third of the space occupied by the convolutions of 

 a single septum at the point indicated. The dorsal lobe is almost twice as 

 long as wide, deeply divided at the lower extremity by a broad, tridentate 

 sinus, each division being again divided into two widely-divergent dentate 



