CRETACEOUS FOSSILS. ' 459 



PTYCHOCERAS CRASSUM. 



Plate 16, figs. 3-5. 

 Ptychoceras crassum Whitf., Prelim. Rept. Pal. Black Hills, 1877, p. 45. 



Entire form of shell unknown. The portion preserved is rather robust 

 in habit and the shell much thickened, except on the ventral surfaces. 

 Transverse section subcylindrical or slightly compressed in a dorso-ventral 

 direction, the larger limb being concave on the ventral side and slightly 

 embracing the smaller part. Tube regularly increasing in diameter with 

 increased growth. Shell ornamented by strong, sharply angular, encircling 

 ridges, which are directed forward in passing from the ventral to the dorsal 

 sides of the smaller limb, but have the opposite direction on the larger 

 part ; also by two longitudinal lines of pointed nodes, or spines, on the 

 dorsum, which are separated by a moderately wide, slightly concave space. 



Septa distant, one septum nearly equaling two of the transverse ridges, 

 except near the bending of the tube, where there are two of the septa placed 

 nearer to each other, beyond which the shell is not septate. Lobes and 

 sinuses of the septa somewhat complicated ; the siphonal lobe longer than 

 wide, and broadly forked at the extremity First lateral lobe large, divided 

 into two principal branches, the outer one being again deeply divided and 

 the margins of the branches serrato-digitate. Second lateral lobe smaller 

 than the first, divided into three short serrate digitations. Antisiphonal 

 lobe small and simple, marked only by two short digitations on each side. 

 First and second sinuses much larger and broader than the lobes, deeply 

 divided in the middle by a small serrated auxiliary lobe, and the divisions 

 each again divided and serrate. Antisiphonal sinus a little smaller than 

 the others, but more deeply divided by the antisiphonal lobe, and the 

 branches again deeply divided. All the margins of the lobes and sinuses 

 are characterized by small rounded serrations. Siphuncle of moderate size, 

 marginal, and situated between the lines of nodes. 



The distinction between this and the preceding species, consists in its 

 large size, more robust habit, and thicker shell, and in the more distant and 

 much more complicated septa. It is still further removed by these charac- 

 ters from P. mortoni M & H., as well as by the simple ridges. 



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

 Woman Fork, Black Hills. 



