124 SYSTEMATIC ARRANGEMENT 



1. ACICULA. 



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. S. 1849, p. 169). Operculum membrana- 

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



1 . G. jamaicensis, Pfeiffer. 



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



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

 behind the tentacles (p. 123). 



Fam. XXV. RISSOELLIDJE. 



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 $* 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. 



