HUDSON-RIVER GROUP. 305 
364. 17. PLEUROTOMARIA [?] BILIX. 
Pu. LXXXIII. Figs. 4 a, b,c, d, e. 
Pleurotomaria bilix. Conranv, Jour. Acad. Nat. Sci. Philadelphia, 1842, Vol. viii, pag. 271, pl. 16, fig. 10. 
Compare Pleurotomaria percarinata, pag. 177, pl. 38, fig. 4, of this volume. 
Obliquely conical ; spire short, composed of four or more volutions, which are somewhat 
appressed above and ventricose below ; last volution somewhat flattened on the lower side ; 
aperture rounded, or slightly transverse surface marked by numerous strong spiral carine, 
which frequently alternate with finer ones; these are crossed by fine strie, which, com- 
mencing at the top of the volution, pass obliquely backwards to the base, or into the 
umbilicus, suffering no alteration of their direction upon the carine. 
This beautiful species seems scarcely appropriately placed under PLevRoromarta, since 
there is no distinct spiral band interrupting the striz, nor a slit in the margin of the aperture. 
For the same reason, also, it cannot fall under the Genus Murcuison1a. This one, with 
some other species, when better known, may constitute a distinct genus; or the Genus 
Hotopea, proposed for some Pleurotomaria-hke shells of the Trenton limestone, may 
perhaps include this one also. 
This shell is readily recognized by the numerous sharp spiral carine which conspicuously 
mark the surface. Between these more elevated ones, there are often regularly alternating 
smaller ones; but these are not always developed. Some individuals, showing but the 
stronger carine, have a very different aspect from others where they are regularly al- 
ternated. The oblique finer striz which cross the carine are often nearly obliterated, and 
scarcely visible to the naked eye. 
Fig. 4a. A small specimen of this species, showing the back of the spire, and the expansion of the 
last volution towards the aperture. 
Fig. 4 6. A portion of the surface enlarged, showing the alternating larger and smaller carine, which 
are crossed by the oblique fime strie. 
Fig. 4 c, A larger specimen — a view from the outside obliquely into the aperture. 
Fig. 4 d. Another specimen, showing the form of the aperture, which is entire on the outer margin, 
Fig. 4 e. The base, viewed in the direction of the spire. 
Position and locality. This species has not been distinctly recognized in the rocks of 
New-York, but it is abundant in the western extension of the same group, associated with 
MopioLopsis, AmMponycuta, and other species, at Madison ( Indiana ), Cincinnati and 
Oxford (Ohio), Maysville (Kentucky ), and near Prairie du Chien ( Wisconsin ). 
365. 18. PLEUROTOMARIA (Species undetermined ). 
Pu. LXXXIII. Figs. 5 a, 0. 
This species has the form of P. lenticularis ; but it is only preserved as casts, which are 
too indistinct to be determined. 
Figs. 5 a, b. Views of the apex and base of two small casts. 
| PaLzonTo.oey.) 39 
