1903.] NATURAL SCIENCES OF PHILADELPHIA. 271 



in rotoedens. The strongly elevated median revolving keel is similar 

 to that of rotoedens, and double, but the nodules are coarser. Length 

 14.5 mm., width 4.5 mm. The corresponding dimensions of an equally 

 well-grown specimen (that is, of six body whorls) of rotoedens, from the 

 Upper Vicksburg, are 11 by 2.8 mm. Arnica may be regarded as a 

 probable ancestor of rotoedens. 



Pleurotoma ancilla n. sp. 



The archetype of tenella in the Red Bluff may be thus named. 

 It is nearly similar in form to tenella but smaller, the nucleus large 

 and well developed, of nearly five whorls, approximately the last two 

 having nmnerous fine acute riblets; it is higher than wide and acute. 

 The subsequent whorls have a broadly obtuse revolving prominence 

 just below the middle, which is closely ribbed, the ribs longitudinal 

 and rounded; collar below the suture consisting of two approximate 

 subequal and slightly uneven revolving lyrse ; space between the collar 

 and median ribbed tumidity moderately expanding and having three 

 or four fine subequal lines; just below the median tumidity there is a 

 fine irregular line. Aperture and canal together short, scarcely more 

 than a third the length of the shell. Length 13 mm., width 4 mm. The 

 specimens measured has about six body whorls. 

 Pleurotoma plutonica n. sp. 



Not rare in the Lower Vicksburg limestones. This species is rather 

 slender, perfectly smooth and polished throughout, with scarcely a trace 

 of revolving sculpture except on the beak, where there are some oblique 

 widely spaced striae. The nucleus is smooth, acutely ogival, higher 

 than wide and of about four whorls. The subsequent whorls have 

 each about eight low rounded oblique ribs, which become obsolete in 

 a revolving concavity below the suture. The first three, or thereabouts, 

 of the body whorls have a rather pronounced, though obtusely rounded, 

 swelling adjoining the suture beneath, but this is gradually lost on the 

 larger whorls, these having but feeble traces of a raised band at the 

 suture, the latter being a very fine, slightly sinuose and feebly 

 impressed line. The canal is well differentiated from the aperture, 

 and the two combined constitute about three-sevenths of the total 

 length of the shell. Length 12 mm., width 3.7 mm. Another speci- 

 men, represented by the spire alone, indicates that the species may 

 attain a length of fully 15 mm. or more. There is no trace of this 

 species in the upper marls. 

 Pleurotoma intacta n. sp. 



Another species, equally well defined, may be named as above. 

 It is small, moderately stout, fusiform, the aperture and canal, which 



