124 SYSTEMATIC ARRANGEMENT 



1. AdCULA. 



Shell subimperforate, rather cylindrical. Peristome rather thick- 

 ened, with subparallel margins joined by a thin callus. Operculum 

 very thin, glassy, few-whorled. 



1. A. fusca. 



2. Geomelania. 



Shell imperforate, turrited ; aperture entire, effused. Peristome 

 simple, expanded ; the margins joined, the basal produced into a 

 more or less developed tongue-shaped appendix. Animal like Aci- 

 cula (A. Adams in P.Z. 8. 1849, p. 169). Operculum membrana- 

 ceous, pellucid, ovate, of a few rapidly enlarging whorls. 



1. G. jamaicensis, Vfeiffer. 



2. G. striata, t. 123. f. 1. 



B. Operculum annular ; nucleus lateral, central. Eyes far back, 

 behind the tentacles (p. 123). 



Fam. XXV. RISSOELLIDtE. 



Rostrum nicked, lobes elongate cylindrical, nearly as long as the 

 tentacula. Central tooth broad, crenulated ; inner lateral broad, 

 crenated ; outer small, hooked. Eyes far back. Operculum half- 

 ovate, annular ; nucleus near the inner straight edge, with a central 

 internal process. — Forbes fy Hanley, B. M. t. J.J. f. 1, 2. 



1. Rissoella. 



1. R. glabra, t. 125. f. 3. 



2. R. diaphana (Jeffreysia d.), Forbes fy Hanley, B. M. t. J.J. f. 1, 



cop. Adams, Gen. Moll. t. 34. f. 6. 



3. R. opalina (Jeffreysia o.), Forbes fy Hanley, B. M. t. M.M. f. 2. 



" Muzzle of Rissoella bifid in front. Tongue with five rows of 

 teeth, 2 • 1 ■ 2 ; central broad, crenulated ; inner lateral broad, crenu- 

 lated ; outer small, hooked. Tentacles two, lateral, subulate. Eyes 

 large, prominent, placed on the back a considerable distance behind 

 the tentacles. Foot oblong, notched and bilobed in front, with a 

 central groove, and slightly rounded behind; operculigerous lobe 

 small and rounded, without filament or appendages. Operculum 

 horny, thin, ovate, annular ; nucleus in the middle of the straight 

 inner edge, with a central internal rib ending in a projecting plate 

 set at right angles with the disk under the nucleus." — Alder. Shell 

 conical. 



Teeth of Rissoella diaphana 3 ■ 1 ■ 3, in eighteen rows ; central 

 broad, rounded above ; apex curved, denticulated ; lateral inner 

 slender, subulate, curved, denticulated near the tip ; outer broader, 

 compressed, curved, denticulated near the end. Prehensile collar 

 broad, triangular, with rows of numerous teeth. Operculum like 

 R. opalina. 



