Casey — Notes on the Pleurotomidae. 167 



into the ribs of the subsequent whorls . The two species in 

 my cabinet may be readily identified as follows : — 



Form slender; shell substance thinner; obtuse periphery of the whorls 

 well above the middle and broadly rounded, scarcely differentiable from 

 the general convexity; lyrae small in size, finer but scarcely more close- 

 set above the periphery; ribs small, somewhat oblique, close-set and 

 numerous, some eighteen in number; callus of inner lip distinct and 

 with a fine free edge throughout; outer lip not distinctly modified 

 within. Length, 4.5 mm. ; width, 1.5 mm. Jacksonian Eocene of the 

 Red River Klmbrel bed jacksonica n. sp. 



Form stouter and with thicker walls, the periphery of the whorls at about 

 the middle and broadly, faintly angulate in profile; lyrae rather strong 

 and more noticeably dilated on the ribs, almost completely obsolete 

 between the latter and much finer above the periphery ; ribs much 

 larger, rounded, longitudinal, about twelve in number; callus of the 

 inner lip almost obliterated for a short distance below the strong 

 prominence at the posterior part of the outer lip; inner surface of the 

 latter prominent with obtuse callus near the sinus and also in a longi- 

 tudinal subbasal ridge, and with one or two minute folds between the 

 two large prominences. Length, 5.0 mm. ; width, 1.8 mm. Jacksonian 

 Eocene of the Montgomery bed. La obesnla n. sp. 



The species described by Meyer,-from the Upper Claiborne 

 sand of Alabama, under the name Mangelia meridionalis, un- 

 doubtedly belongs to this genus. It differs from the species 

 above described in having two rounded and two carinated 

 smooth embryonic whorls and five body whorls, and even with 

 this number of whorls, which may be a mistake of the de- 

 scriber, the figured type seems to be immature, as the colum- 

 ellar folds do not appear and the outer lip is not of an adult 

 type ; it is materially larger than either of the species de- 

 scribed in the table. This genus represents the oldest type 

 of non-operculate Pleurotomidae known to me at present. 



Helenella n. gen. 



This generic name is proposed for certain very small spe- 

 cies apparently confined to the fauna of St. Helena, though 

 possibly extending to the entire West African f aunal province 

 and having a form of embryo wholly different from any- 

 thing else in the tribe. The shell is oval or fusiform, mod- 

 erately thick in substance, closely, spirally nodulose in 



