HUDSON-RIVER GROUP. 
305 
364. 17. PLEUROTOMARIA [ ? ] BILIX. 
Pl. LXXXIII. Figs. 4 a, b, c, d, e. 
Pleurotomaria bilix. Conrad, 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 carinse, 
which frequently alternate with finer ones ; these are crossed by fine striae, 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 carinse. 
This beautiful species seems scarcely appropriately placed under Pleurotomaria, since 
there is no distinct spiral band interrupting the striae, nor a slit in the margin of the aperture. 
For the same reason, also, it cannot fall under the Genus Murchisonia. This one, with 
some other species, when better known, may constitute a distinct genus ; or the Genus 
IIolopea, proposed for some Pleurotomaria-like shells of the Trenton limestone, may 
perhaps include this one also. 
This shell is readily recognized by the numerous sharp spiral carinse 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 carinse, have a very different aspect from others where they are regularly al¬ 
ternated. The oblique finer striae which cross the carinse are often nearly obliterated, and 
scarcely visible to the naked eye. 
Fig. 4 a. A small specimen of this species, showing the back of the spire, and the expansion of the 
last volution towards the aperture. 
Fig. 4 b. A portion of the surface enlarged, showing the alternating larger and smaller caring, which 
are crossed by the oblique fine striae. 
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 
Modiolopsis, Ambonychia, and other species, at Madison ( Indiana ), Cincinnati and 
Oxford ( Ohio ), Maysville ( Kentucky ), and near Prairie du Chien ( Wisconsin ). 
365. 18. PLEUROTOMARIA (i Species undetermined). 
Pl. LXXXIII. Figs. 5 a, b. 
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. 
[ Palaeontology.] ' 39 
