45 



and,:JiiesLted tolliV^^^^^^^^^^ with P^ycWaslfor^oni M. & H., 

 ««i«f;^f /!■<¥ ponsider them as distinct: but there are so many 



^^l«fhlf f T'; ^""^ *^" variations are so great, that it appears im- 

 possible 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 structure and location of the septa. We are qiiite 

 well aware that this latter feature is one liable to great variation, de- 

 pendant 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. Beside 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 Pierr6 group, on 

 Beaver Creek, Black Hills. 



Ptychoceras cbassum, n. sp. 



Plate 16, flgs. 3-5. 



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- ven- 

 tral 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 oppo- 

 site 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 sep- 

 tate. Lobes and sinuses of the septa somewhat complicated; the 

 «iphonal 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 

 muQh 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 



