44 



deeply divided above and strongly lobed on the sides. Second sinus 

 smaller, less deeply and unequally divided, the outer half smallest and 

 deeply iobed, the other less deeply so. Antisiphonal sinus very large 

 (considering the parts separated by the antisiphonal lobe as one sinus), 

 the antisiphonal lobe dividing it nearly to the base, and the divisions 

 again very deeply divided and deeply lobed on the sides. Siphuncle 

 small, situated between the lines of nodes and marginal. 



This species differs from Ancyloceras Jenneyi in being sinistrally 

 coiled, and in the character of the ridges of the surface and angular 

 nodes. We know of no other species described with which it is enough; 

 related to require a comparison. 



Formation and locality. — In limestone at the top of the Fort Pierr© 

 group, on the Cheyenne, near French Creek, Black Hills, Dakota. 



Ptyohookras meekantjm, n. sp. 



Plate 16, figs. 1 and S. 



Shell of small size, subcylindrical or vertically compressed, giving a 

 somewhat oblate-transverse section, very gradually increasing in diam- 

 eter with the increased length ; smaller limb slender, gently curved or 

 bent in the middle of its length, so as to give the earlier and middle 

 portions of the shell an angle of about 135° to each other. Larger 

 limb or outer chamber closely appressed and slightly embracing the 

 smaller, and extending to near the middle of the curvature. 



Surface of the entire shell marked by strong, simple, sharply angular^ 

 oblique, encircling ridges, with wider, deeply concave interspaces, very 

 regularly increasing in distance ffom the apex outward, and on the 

 dorsal region by two longitudinal rows of spines or spiniform tubercles, 

 situated at about or nearly one-sixth of the circumference of the tube 

 from each other, and on the crests of the ridges. The encircling ridges 

 are directed obliquely forward in passing from the ventral to the dorsal 

 surface in the smaller limb, but have the opposite direction on the 

 larger ; 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 digitations. 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 division being 

 marked by short rounded digitations. 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 but simple in the third ; 

 division of the sinuses marked by short rounded digitations. Siphont 

 of moderate size, marginal, and placed between the lines of nodes. 



