458 GEOLOGY OF THE BLACK HILLS. 



smaller limb, but have the opposite direction on the larger limb; space 

 between the lines of nodes very gently concave. 



Septa approximate, but more distant than the surface ridges of the shell, 

 deeply lobed and the lobes and sinuses somewhat complicated. The dorsal 

 or siphonal lobes are nearly as wide as long, broadly forked at the extremity, 

 and the outer side of the forks marked by several short, rounded digita- 

 tions. First lateral lobe divided into two diverging branches, with lobed 

 margins. Second lateral lobe narrower and more slender than the first, and 

 more deeply forked; the divisions being marked by short, rounded digita- 

 tions. Ventral, or antisiphonal lobe small, but deeply forked, and the 

 margins sinuous Sinuses nearly equal in size, each of them deeply divided 

 in the middle by an auxiliary lobe; slightly bifid in the first and second 

 sinus, but simple in the third ; division of the sinuses, marked by short, 

 rounded digitations. Siphon of moderate size, marginal and placed between 

 the lines of nodes. 



Before entering upon a critical study of the specimens before us, we 

 had supposed them to be identical with Ptychoceras mortoni M. & H , and 

 hesitated to consider them as distinct ; but there are so many points of dif- 

 ference, and the variations are so great, that it appears impossible to avoid 

 this conclusion, and equally difficult to unite the two forms here mentioned. 

 More especially is this the case if any reliance is to be placed on the struc- 

 ture and lobation of the septa. We are quite well aware that this latter 

 feature is one liable to great variation, dependent upon the growth of the 

 individual ; but the lobations in the septa at the smaller extremity of the 

 specimen figured is much more complicated than that shown by Mr. Meek 

 to exist in the type specimen of P. mortoni. Besides the difference in the 

 septa, the form of the tube varies in being wider than high, the reverse of 

 that species, and the antisiphonal surface of the tube is always flattened, 

 and the costa always simple. 



Formation and locality. — In limestone of the Fort Pierre* Group, on 

 Beaver Creek, Black Hills. 



