Casey — Notes on the Pleurotomidae. 153 



Tropisurcula n. gen. 



The shell hi this genus is small in size, of slender form, 

 with moderately elongate and slender beak and narrow, 

 closely coiled, multispiral embryo, which is higher than wide, 

 and perfectly smooth and polished throughout. The ribs 

 are about equal in number to those of Pleurofusia but cul- 

 minate in transverse prominences at the summit of an angu- 

 late median periphery, and sometimes become gradually 

 feebler on the larger whorls. The spiral lines are subequal 

 among themselves, relatively rather coarse but low, and very 

 close-set throughout. The species known to me are two in 

 number as distinguished by the following characters : — 



Ribs nine to ten in number, rounded and distinct throughout the length of 

 the whorls, though much more prominent on the peripheral ridge and 

 tending to become extinct on the broad fasciolar surface of the larger 

 whorls; spiral lines coarse but not much elevated, even above, coarser 

 and usually separated by a fine line below, the periphery; apex of 

 the embryo obtuse. Length of a specimen of 5 body whorls, 10 

 mm.; width, 2.9 mm. Red Bluff Eocene to the Vicksburg 

 Oligocene caseyi Aid . 



Ribs seven or eight in number, more broadly rounded, very promineut on 

 the angulate periphery, becoming rapidly obsolete below, and usually 

 wholly effaced on the fasciolar surface above, the periphery, especially 

 on the larger whorls; spirals rather wide but feebly elevated, close-set 

 and subequal throughout, but still feebler on the fasciolar surface ; 

 embryo acute at tip, of five smooth and highly polished whorls, narrow- 

 ing more rapidly above the two basal whorls, the apical very small. 

 Length of a specimen of 1 body whorls, 7.5 mm.; width, 2.5 mm. 

 Lower Claiborne Eocene of St. Maurice, La . . crennla n. sp. 



The embryo in caseyi is much more evenly and feebly 

 tapering from base to apex than in crenula. The former 

 species was described by Mr. Aldrich under the generic name 

 Drillia . 



Surculoiua n. gen. 



The type of this genus was regarded by Cossmann as a 

 member of his genus Amblyacrum, and, while the small pauci- 

 spiral embryo is very nearly the same in general form, all the 

 other characters differ to so great a degree that it is not easy 



