268 PKOCEEDINGS OF THE ACADEMY OF [Feb., 



at least two species in the Vicksburg, both confined to the lower hori- 

 zon. 



There are several species of Pleurotoma in the Vicksbiirg and sub- 

 adjacent horizons allied more or less closely to servata Con., and appar- 

 ently neglected or overlooked hitherto in our literature, but which can 

 be distinguished readily by brief comparative descriptions. In servata 

 the nucleus is smooth, elevated, acute and of 3 or 4 whorls, the last 

 whorl gradually acquiring a few riblets which become by degrees the 

 7 or 8 large rounded ribs of the body whorls. There is no appearance 

 of a subcentral revolving carina on the whorl adjoining the nucleus, 

 and on each of the more recent whorls there are generally 3 coarse 

 revolving lines thickened on the ribs, with numerous very fine close-set 

 intermediate threads, all occupying about lower half of the whorl. Just 

 below the suture there is a conspicuous thickened collar, immediately 

 below which there is a deep revolving concavity, the surface thence 

 expanding to the uppermost of the coarse revolving lines, the entire 

 surface between the latter and the collar having fine subequal and 

 rather close-set lines. The aperture and canal together constitute 

 about three-sevenths of the total length of the shell. 

 Pleurotoma vicksburgensis n. sp. 



This species occurs plentifully in the Vicksburgian beds, accompany- 

 ing servata and generally confounded with it. It usually attained a 

 a little larger size and stouter form, and may be distinguished at 

 once by the fact that the whorl immediately adjoining the nucleus has 

 a strong revolving line below the middle, thickened on the ribs and 

 accompanied by a close-set smaller revolving line immediately above 

 it. The larger whorls generally acquire two other coarse, though much 

 smaller revolving lines, one above and one below the two mentioned, 

 and also finer intermediate threads. Just below the suture the 

 elevated collar is not quite so prominent as in servata, and, instead 

 of the abrupt concavity adjoining, the surface is almost evenly con- 

 cave and rapidly expanded to the system of coarse revolving lines 

 referred to, this surface being also finely, evenly lyrate. The nucleus 

 is much shorter than in servata, consisting of between two and three 

 whorls, and is not higher than wide. The aperture and canal are 

 nearly as in servata. One of the larger specimens before me measiu^es 

 27 mm. in length by 7 mm. in width. The double carina of the nepi- 

 onic whorls remains throughout the most conspicuous feature of the 

 revolving sculpture, the lines becoming gradually more nearly equal 

 and more widely spaced, with the dilatations on the ribs much more 

 pronounced than in servata; the ribs, also, are much more broadly 



