518 PALEONTOLOGY OF ILLINOIS. 



case, it is nearly as strongly plicated as that shell in its more regular, 

 less attenuate form and rougher surface, as well as in not having its 

 folds or plications forming five regular, broad ridges more or less flat- 

 tened and concave along their middle. 



From P. fissurella, Hall, this species differs in being less depressed, 

 or more attenuate, particularly near the apex, which is never oblique, 

 as in that species. 



Prof. HALL had described this species in 1860, but as he, by an over- 

 sight, gave it the same specific name (siibrectum) he had previously 

 applied to another species from the Upper Helderberg rocks of New 

 York, it became necessary, in order to prevent confusion, that our Illi- 

 nois shell should receive another name, and hence we proposed to cnll 

 it P. infundibulum. 



From the same locality and position with the above, we have a single 

 specimen, differing from the others in being greatly more slender and 

 elongated. It is perfectly straight, somewhat compressed laterally, and 

 about twice as long as wide, being very attenuate above the middle, 

 and but moderately expanded below. It is an internal cast, showing 

 no surface markings, but preserving the transversely elongate-oval 

 muscular scar on each side, apparently connected by a slender band 

 passing around behind. Without more specimens, it is not possible to 

 determine beyond doubt whether this is a distinct species, or only an 

 elongate varity of the P. infundibulum. Should it prove to be distinct, 

 we have proposed, in the paper cited at the head of this description, to 

 call it P. (Ortlionychia) extinctor, from its resemblance in form to a 

 candle extinguisher. It is represented by figures 3, <Z, e, of plate 17. 



Locality and position. Keokuk division of the Lower Carboniferous 

 series, at Warsaw, Illinois. 



PLATYCERAS EQUILATERA, Hall. 



PI. 17, Fig. 2. 

 Platyceras equilatera. HALT,, I860. Supplementary sheet to Vol. I, Part II, Iowa Report, p. 1. 



SHELL attaining a medium size, composed of about one 

 to one and a half turns ; appex small, laterally compressed, 

 and closely incurved, nearly on the same plane as the gen- 

 eral curve of the body of the shell, or but very slightly ob- 

 lique; body portion merely arched, and rapidly and nearly 

 equally expanding to the aperture, which has an irregular, 

 sub-circular, or broad sub-oval, outline ; lip rather sharp, and 



