MUSEUM OF COMPARATIVE ZOOLOGY. 279 



opalescent white, with an unpolished surface; the earlier whorls show minute 

 Scala-\ike transverse ribbing, the later ones are smooth except for silky incre- 

 mental lines; last whorl much the largest; aperture rounded ovate, margin 

 simple, sharp; columella straight, forming the front edge of a very deep nar- 

 row vertically striated umbilicus; umbilicus with a strong serrate carina 

 which does not angulate the margin of the aperture. Alt. 2.75 ; max. diam. 

 2.50 mm. 



Habitat. U. S. Fish Commission Station 2610, off the North Carolina 

 coast, in 22 fins., gravel, bottom temperature 79°. F. 



This species is related to A. (C.) depressus Seguenza, but has not the beau- 

 tiful surface shagreening of that species and is rounder, fuller, and generally 

 more compact and naticiform. 



Family RISSOID^E. 



Genus RISSOA Freminviixe. 



A vast number of small shells are included under this genus. Without 

 doubt a reasonable classification of them will render a separation into sub- 

 genera and sections advisable and convenient. But many of the attempts to do 

 this have been so bungling, and the naming of subordinate groups has been 

 carried to such an excess, as practically to oblige one who has not especially 

 studied the family either to select unguided from a chaos of names or to fall 

 back upon the original generic name. As I have only two species from the 

 Blake collection to consider, I prefer the latter course. The arrangement 

 adopted by Mr. Watson in the Challenger Report is clear and rational; it is to 

 be hoped that the subject will be taken up by some conservative person, not 

 afraid of drudgery, and permanent order brought out of the present chaos. 



Rissoa precipitata n. s. 



Plate XIX. Fig. 1. 



Shell conical, blunt, thin, translucent or milky-white, with four and a half 

 whorls; nuclear whorl and a half depressed, large, opaque white, smooth, 

 polished; remainder polished, transversely sculptured with about twenty-two 

 close-set well-defined ribs, extending from suture to suture, and on the last 

 whorl to the periphery, beyond which they become obsolete or merge into the 

 lines of growth; just before the suture is an impressed line or constriction, be- 

 hind which on the later whorls the ribs are nodulous; near the periphery the 

 shell is marked by more or less distinct grooves, and where the intercalary 

 ridges are intersected by the ribs there is often a slight nodulation; these 

 grooves and approximately equal ridges between them are evident over most of 

 the rounded base; aperture rounded in front, angulated behind; pillar with a 

 slight callus and an (accidental?) minute perforation behind it; outer lip thin, 



