276 PROCEEDINGS OF THE ACADEMY OF [Feb., 



MICRODRILLIA n. gen. 

 A number of minute Pleurotomids, including injans and cossmanni 

 of Meyer, and harrisi of Aldrich, have been referred to by Cossmann 

 under the namas Asthenotoma and Scobinella, by Harris under Mangilia, 

 by Aldrich under Glyphostoma, and by Meyer, Vaughan and others 

 under Pleurotoma in its broad sense. They are all very small and 

 characterized by a well-developed, multispiral, closely coiled embryo, 

 having one to three of its basal whorls costulate, few body whorls 

 which are wholly devoid of costse but spirally carinate, the retral 

 sinus relatively large, circularly rounded and close to the suture, 

 the aperture oblique, columella callous, with or without plications, 

 and the canal short or subobsolete. 



The genus Microdrillia differs from Asthenotoma, to which cossmanni 

 was referred by Cossmann, in the structure of the embryo, and, espe- 

 cially, in the position of the retral sinus, which in Asthenotoma corre- 

 sponds in its greatest depth with the median line or periphery of the 

 whorls. In fact, there is only one American species known to me 

 which can properly be assigned to Asthenotoma, this being the PI. 

 texana of Gabb. Microdrillia is much more closely related to Glypho- 

 stoma, as suggested by Aldrich, but is not at all allied to Mangilia. 

 It appears to have become wholly extinct in the Oligocene or Lower 

 Miocene. The species were numerous and individually abundant, 

 especially in the mid-Eocene of the Lower Claiborne, and those before 

 me may be readily identified by the following table : 



Columella without folds, 2 



Columella with numerous rather widely and evenly spaced folds; shell thick and 

 heavy, the base angulate, not at all rostrate 8 



2 — Shell rhomboidal in profile, thick and strong, the ante-peripheral part but 

 little shorter than the entire portion behind the periphery of the body 

 whorl; revolving carinae very thick 3 



Shell more elongate, the ante-peripheral part always much shorter than the 

 post-peripheral, thinner and more delicate, with relatively fine carina^. . 4 



3 — Embrj'o small, evenly and broadly conical, of three smooth and one finely 

 costulate whorls ; body whorls four in number, each with three strong carinsp, 

 the lowermost carina first appearing generally on the second or third whorl; 

 lines of growth strongly marked, cancellating the body whorl; columella 

 subumbilicate in the type. Length 4.5 mm.; width 2 mm. Jacksonian 

 Eocene of Moody's Brancli, Miss., . . . [Plewotoma] cossmanni Meyer 



[weycri Coss.] 



Embryo similar in structure I)ut much larger; body whorls generally not more 

 than three in number, each with two very thick carinse; columella not um- 

 bilicate in the specimens at hand; lines of growth very fine, scarcely notice- 

 able. Length 2.8 mm., width L4 mm. Lower Claiborne Eocene. A^esey 

 Creek, Lee Co., Tex solidula, n. sp. 



