FOSSILS OF THE KEOKUK GROUP. 519 



more or less sinuous, sometimes distinctly so. Surface with 

 undulating lines, and near the margins of the lip, stronger 

 sub-imbricating marks of growth ; the undulations in the 

 markings corresponding to the sinuosities of the lip, which 

 sometimes produce traces of obscure longitudinal folds near 

 the aperture. 



Greatest length of a mature specimen, measuring from 

 the anterior margins of the lip to the most prominent part 

 of the arch of the spire, 1.62 inches; breadth of the aper- 

 ture, 1.15 inches. 



Like many other species of this genus, this shell varied considerably 

 at different stages of growth, the young shells being nearly smooth, 

 while in adults the undulating marks of growth are strongly defined 

 near the lip, which often becomes strongly sinuous, particularly on the 

 anterior lateral margins. In one specimen (fig. 2 c, pi. 17.) there are two 

 deep anterior sinuses, with a prominent linguiforin extension between. 

 This individual has much the appearance of P. trilobum (=Pi1eopsis tri- 

 lobus. Phillips,) to which the species is evidently closely allied. 



Locality and position Keokuk division of the Lower Carboniferous 

 series ; Keokuk, Iowa, and Warsaw, Illinois. 



PATYCERAS FISSUKELLA, Hall. 



PL 17, Fig. 4. 



Platycereu Jt&ureUa, HALL, 1860. Sheets entiUed "Supplement GeoL Report Iowa, Vol. I, Part n, 

 P- 2, : 



SHELL rather thick, obliquely depressed conical, expand- 

 ing very rapidly from the apex to the base; posterior side 

 short, the descent from the apex being steep, or sometimes 

 nearly vertical, and concave in outline ; lateral slopes 

 gtraighter; anterior slope longest, and generally convex in 

 outline; apex arching backwards a little, but showing no 

 tendency to a spiral curve, somewhat obtusely pointed, and 

 placed between the middle and the posterior side, or some- 

 times nearly over the latter; aperture very large, sub-circu- 

 lar or a little oval, the length being generally somewhat 

 greater than the breadth; lip beveled and a little uneven 



