43 



the terminations of the lobes bipratite instead 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 errone- 

 ously 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. 



AXCYLOCERAS TRICOSTATUS, n. 8p. 



Plate 15, fic?s. 7 and 8. 



A single fragment of a sinistra, 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 dis- 

 tinct from those of any described species. The form of the volution has 

 been quadrangularly ovate in section, vertically compressed, with the 

 greatest vertical thickness 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 for- 

 ward 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 flattening, and are recurved below 

 and on the umbilical surface. The costa 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 space 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 branches. There are also two lateral dentate spurs 

 above on each side of the lobe. First lateral lobe large, but slender > 

 divided into two principal widely divergent branches, each of which is 

 again divided and furnished with several strong dentate spurs on the 

 margins; two other spurs of similar character exist on each side of the 

 lobe, one just below the bifurcation, and the other some distance above. 

 Second lateral lobe smaller than the first, deeply divided, with the 

 branches less divergent, but again divided, and the inner branches tri- 

 furcate, the outer branches short and strongly dentate. Autisiphonal 

 lobe small and simple, marked by one small spur on each side above, 

 and two larger curved and dentate ones below ; the central termination 

 being minutely tridentate at its extremity. First lateral sinus large, 

 very deeply divided into two principal branches, each of which is 



