38 SYNOPSIS OF THE CHARACTERS OF THE 



EUOMPHALUS TABULATUS. P]dl. SP. 

 Cirrus tabulatus. Pliil. Geol. York. 



Sp. Ck. — Obtusely conical, depressed; summit of the spire flattened; whorls with a nearly rectangular 

 keel in the middle, separating the upper half, which is flat or concave, from the lower, which is flattened, and 

 slightly oblique ; mouth wider than long. 



This species is remarkable for the square form of its whorls, which are plain or tabulate above, rounded 

 beneath, and widely umbilicate ; they are usually about five in number ; the upper portion of the spire seems as 

 if truncated. Diameter two inches, height ten lines, thickness of last whorl five lines. 



Platyschisma. M' Coy". 



Gen. Ch. — Obtusely conical, ventricose ; shell very thin ; spire short, obtuse, of few whorls ; aperture large, 

 lunate, rounded anteally, narrow retrally, with a very wide, shallow sinus in the outer lip, not forming a defined 

 band ; surface smooth, or only marked by lines of growth ; umbilicus small, roimded. 



This very distinct group of PleurotomaricB difi'ers from all the Oolitic, and the other Palaeozoic forms, in seve- 

 ral important characters, of which, the want of a definite mesial band is one of the most striking : they are also 

 recognized by their obtuse form ; smooth siu-face, without keels of any kind ; the great width and shallowness 

 of the sinus, as indicated by the lines of growth ; and the thinness of their shells. They resemble the recent 

 Janthinm strongly, but difier from them in having the mouth rounded anteally instead of being angulated, and 

 not having the pillar lengthened, as in that genus. The species appear confined to the carboniferous limestone. 



Platyschisma cirroides. M'-Coy. (PI. VI. fig. 2). 



Sp. Ch. — Shell discoid, depressed, umbilicate; base flattened, or slightly concave; whorls five, very gra- 

 dually increasing, the last turn being but little larger than that which precedes it ; slightly convex ; body whorl 

 obtusely carinated on its lower edge, spire very obtuse. 



A very fine species, resembling the P. helicoides in some particulars ; it is, however, a much rarer shell, 

 and easily to be distinguished by the characters above mentioned. Diameter two inches three lines ; height of 

 spire one inch three lines. 



Platyschisma helicoides. Sow. sp. 



Ampullaria helicoides. Sow. — Pleurotomaria helicoides. Phil. 



Sp. Ch. — Ovate, depressed; spire obtusely pointed, of four slightly convex whorls; base tumid; aperture 

 lunate, with an obtuse sinus in the middle of the outer lip, wliich is followed by the lines of growth ; umbilicus 

 small, deep, rounded, spirally striated witliin. 



This fine species is not uncommon ; the shell is excessively thin ; and from the wide, shallow, rounded 

 sinus, it seems more nearly allied to Janthina than either Ampullaria or Pleurotomaria. 



Platyschisma Jamesii. M'Coy. (PL V. fig. 20). 



S]>. Ch. — Depressed, conical; spire obtusely pointed, of four or five flat, imbricating whorls; slightly tumid 

 at the sutures ; base flattened, rounded at the circumference ; umbihcus small, surface marked with sharp, oblique 

 strise, having a broad, rounded, backward wave on the rounded circumference of the last whorl, and a little 

 above the suture on the turns of the spire. 



This is one of those ambiguous little shells, most allied to Trochus, and having much of the general cha- 



^ TlXxTVi, wide ; and <r^i<rfi>i, a slit. 



