120 BULLETIN OF THE 



variety formosa has the sculpture stronger, and is a little thicker; the nucleus 

 intermediate between the type and the variety Benedicti. The latter has the 

 sculpture somewhat sharper, and is more elevated proportionally, the nucleus 

 being taller and with one more whorl. The differences are not greater than 

 those observed in other cases, both among the larval and the adult shells. I 

 have compared typical specimens of all the named forms. The height and size 

 of the larval shell depends upon its original size in the ovicapsule, where, it is 

 well known, some individuals are much larger than others, and upon the length 

 of time the animal continues in the larval state, which differs according to 

 temperature and other factors of the environment. The differences spoken 

 of by Jeffreys in the nucleus of his Defrancia formosa are well illustrated by 

 the specimens in his collection, which are fully as variable as the adult shells, 

 and, as far as depressed or elevated form is concerned, much more discrepant 

 among themselves. In this connection I would refer to my introductory 

 remarks, Part I. p. 183. 



Pleurotomella leucomata Dall. 



Plate XI. Fig. 13. 



Pleurotoma {Drillial) leucomata Dall, Bull. M. C. Z., IX. p. 63, 1881. 



Shell brilliant white, with a trochoid brown glassy nucleus of four whorls, the 

 last of which has a sharp peripheral keel ; subsequent whorls seven, with a 

 peripheral row of (on the last whorl 14) short prominent subnodular waves 

 which on the last two whorls tend to become a little elongated and oblique ; 

 they do not form ribs or pass on the last whorl before the periphery; between 

 them and the suture the whorl is excavated somewhat and marked with very 

 fine spiral grooves crossed by the arched incremental lines ; over the nodules 

 run a few small spiral threads, strongest on the early whorls ; on the base are 

 five or six flattened threads with wide interspaces, on the canal are about a 

 dozen smaller threads, closely set ; the whole surface appears polished and the 

 fasciole smoooth ; notch deep and wide, abutting on the preceding whorl at 

 the suture ; outer lip arched forward about a quarter of a turn, thin, simple ; 

 inner lip smooth, pillar short, obliquely twisted; canal wide, distinct, flaring at 

 the end, slightly recurved ; aperture narrow, shorter than the spire. Max. Ion. 

 of shell, 13.75 ; of last whorl, 8.2 ; lat. of shell, 6.0 mm. 



Habitat. U. S. Fish Commission Station 2384, in the Gulf of Mexico be- 

 tween the delta of the Mississippi and Cedar Keys, Florida, in 940 fms., mud, 

 temperature, 39°.6 F. Blake Station 48, in 533 fms., Gulf of Mexico, bottom 

 temperature, 41°. 75 F. 



Though small, this is one of the most elegant of the American species, and 

 does not accord in the details of its sculpture with any other known to me. 



