160 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 58. 



culigerous lobe small and rounded, without filaments or prolonged appendages. 

 Foot oblong, notched and bilobed in front, with a groove down the center, and 

 slightly rounded behind. The armature of the tongue consists of a broad 

 crenuluted central tooth, flanked by two lateral ones on each side — the first 

 broad and crenulated, the exterior one small and hooked. 



There is no character in the shell of this curious genus by which it can be dis- 

 tinguished from B.issoa. In the only two species yet known, both minute, the 

 shell is transparent, and from the remarkable position of the eyes of the animal, 

 so far behind the usual place, and constantly within the shell, its transparency 

 is probably a constant character of the genus, being necessary for the exercise 

 of vision. The lower tentacles may be considered to represent the lobes of the 

 muzzle in Rissoa, here elongated into tentacles and covered with vibratile cilia 

 in the same manner with the upper pair. These latter are more flattened and 

 broader than in Rissoa. 



The operculum is very peculiar. The projecting internal plate I do not recol- 

 lect to have observed in any other genus, though the spine in Nerita approaches 

 to it. It appears from the ridges on its inner surface to afEord attachment to a 

 muscle. 



Jeffreysia is a littoral genus, found in company with Rissoa on small seaweeds 

 in pools between tide marks. Its alliance is evidently with that genus, which 

 in the shell it so strongly resembles; and the lingual armature bears out the 

 afBnity, differing but little from that of Rissoa internipta and some of the 

 commoner species. Some others of the small transparent shells usually in- 

 cluded under Rissoa may probably, when they are obtained alive or with the 

 operculum, be found to belong to this genus. 



Four West American species are now referred to this genus. Of 

 two of these the operculum is known so their status may be con- 

 sidered without question. These are Je-jfreysia hifasciata Carpenter 

 and Jeffreysia tumens Carpenter, both of which were described in 

 the Mazatlan Catalogue in 1856. The third species, Rissoa anguli- 

 ferens de Folin, described in Fonds de la Mer in 1870 (vol. 1, p. 

 134), is placed here provisionally. It seems to belong here, but we 

 have seen no specimens, and de Folin does not describe the opercu- 

 lum; its status, therefore, requires confirmation. Of the fourth 

 species, here described as new, the operculum is also unknown. Its 

 general shape and peculiar umbilicus would place it near Rissoella 

 tumens Carpenter. 



KEV TO TUB WEST AMEIUCAN UISS0KLL.1.S. 



Shell umbilicated. 



Shell smooth tumens. 



Shell not smooth but axially threaded excolpa. 



Shell not umbilicated. 



Periphery angulated angulif evens. 



Periphery not angulated hifasciata. 



RISSOELLA TUMENS (Carpenter). 



Plate 12, fig. 1. 



1856. Jeffreysia tumens Carpenter, Cat. Maz. Shells, p. 363. 



1857. Jeffreysia tumens Carpenter, Kept. Brit. Ass. Adv. Sci. for 1856, 



pp. 257. 327. 



Shell small, subglobular. openly umbilicated, with a slender thread 

 bordering the umbilicus ; thin, white, diaphanous, smooth. Nuclear 



