HUDSON-RIVER GROUP. 



305 



- 364. 17. PLEUROTOMARIA[?] BILIX. 



Pl. LXXXIII. Figs, i a, b, c, d, e. 

 Plenrotomaria bilix. Conrad, Jour. Acad. Nat. Sci. Philadelphia, lS-12, Vol. viii, pag. 21\, pi. 16, fig. 10. 

 Compare Pleurotomaria percarinata, pag. 177, pi. 38, fig. 4, of this volume. 



Obliquely conical ; spire short, composed of four or more voluiions, wliicli are somewhat 

 appressed above and ventricose below ; last volution somewhat flattened on the lower side ; 

 aperture rounded, or sliglilly transverse ; surface marked by numerous strong spiral carinae, 

 which frequently alternate with finer ones ; these are crossed by fine stri<c, which, com- 

 mencing at the top of tlie volution, pass obliquely backwards to the base, or into the 

 umbilicus, suffering no alteration of their direction upon the carinjc. 



This beautiful species seems scarcely appropriately placed under Pleurotomari.\, 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 

 HoLOPEA, 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 carinae 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 carina?, have a very different aspect from others where they are regularly al- 

 ternated. The oblique finer striae which cross the carinae 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 i. A portion of the surface enlarged, showing the alternating larger and smaller carinae, which 



are crossed by the oblique fine stria;. 

 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. IS. PLEUROTOMARIA {Species undetermined) . 



I'L. 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. 

 [ Paleontology.] 39 



