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, 4.5mm.; width, 1.5 mm. Jacksonian Eocene of the 
Red River Kimbrel bed........essscscscsecvcescovees jacksonicea pb. 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,1.8mm. Jacksonian 
Eocene of the Montgomery bed, La..................--.obesula 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 nh. 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 faunal 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 
