NATURAL SCIENCES OF PHILADELPHIA. 271 



Locality and position. St. Genevieve Co., Missouri; Chester division of the 

 Siibcarboniferojis series. 



Platyschisma hemcoides, Sowerby? (sp.) 



The specimens before us agree so exactly with the figures and descriptions 

 of Sowerby's Ampullaria (Globulus) helicoides, from the English Mountain lime- 

 stone, that we are completely at a loss to find any characters by which it can 

 be distinguished. The largest of them are somewhat smaller than the aver- 

 age size of English specimens, and none of them are so depressed as the form 

 for which Phillips proposed the name Natica elongata ; their outline being 

 more nearly like Sowerby's fig. 2, pi. 522, Min. Con. On comparison with 

 specimens of the Belgian form from Tournay (usually referred to Sowerby's 

 species), which they never equal in size, and which seem to us probably dis- 

 tinct from the English species, they are found to differ in having the whorls 

 less rounded above, and the revolving strife within the small umbilicus coarser. 

 The surface is quite smooth, the apex rather obtuse, and some of the speci- 

 mens show indications of the faint sinus in the outer lip, which has caused the 

 European specimens to be sometimes referred to the genus Pleurotomaria. 

 There are no traces of a spiral band, however, and some individuals seem to 

 have had no notch or sinus in the lip. 



Locality and position. Chester limestone, of the Subcarboniferous series, St. 

 Genevieve Co , Missouri ; where it is quite abundant, aud occurs with a Nau- 

 tilus (Trematodiscus) we cannot distinguish from iV". sulcatus, Sowerby. 



Pleurotomaria conoides, M. & W. 



Shell small, regularly conoid-trochiform. longer than wide, the breadth be- 

 ing to the length about as five to six. Volutions five or six, increasing regu- 

 larly and rather gradually in size, all obliquely flattened nearly parallel 

 to the slope of the spire, though the lower margin of each projects at i lie 

 suture slightly beyond the upper edge of the succeeding one below ; last one 

 angular around the periphery at the base, and flattened on the under side at 

 less than a right angle to the oblique slope above, but rounding abruptly into 

 the minute umbilical perforation within. Aperture rhombic quadrangular, 

 with nearly equal length and breadth ; inner lip straight and parallel to the 

 axis of the shell below, but curving out abruptly at its base. Surface orna- 

 mented with small, regular, oblique, arching s f ria3 on the upper sloping sides 

 of the whorls, and minute sigmoid lines, crossed near the periphery by faint 

 traces of a few revolving stria?, on the under side of the body whorl. Spiral 

 band narrow, located at, or slightly above the periphery of the body volution, 

 and passing around its own breadth above the suture on the whorls of tbe 

 spire ; margined above and below by a raised line. 



Length, 0-27 inch ; breadth, 0'23 inch ; apical angle regular, divergence 

 about 50. 



This species belongs to the trochiform section of the genus, including Pleu- 

 rotomaria obtusispira, and P. Riddellii, Shumard ; P. turbiniformw, M. & W., 

 and P. Missouriensis, Swallow, (sp.) It differs from all these shells, however, 

 in being much smaller, although composed of about the same number of 

 whorls ; while it also differs from them all excepting the P. obtusispira in hav- 

 ing no revolving striae on the upper side of its whorls, and from that species 

 in having a more elevated spire, and rather coarse, instead of " extremely fine, 

 stria? of growth " on the upper slope of its whorls. In form and general ap- 

 pearance it resembles quite nearly Trochus coniformis, de Koninck (An. Foss. 

 pi. xxxvii. fig. 4, a, b,)* but differs in wanting the spiral striae, and of course 

 in the possession of a distinct, but narrow spiral band. 



*This snell resembles so closely in form, surface markings and general outliue, several of our 

 American Carboniferous species of Pleurotomaria, that in case it had been described by a lees ex- 

 perienced palaeontologist than I'rof. de Koninck, we should have suspected it to belong to that 

 genusinstead of being a true Trochus. In one Pleurotomaria turbiniformis, for instance, and tbe 

 beautiful species described by Prof. Swallow under the name Trochus Missouriensis, the spiral 

 band is so very narrow and inconspicuous as to be easily overlooked, when the margin of the lip 

 is broken away. 



1866.] 



