98 SYSTEMATIC ARRANGEMENT 



Operculum of Rissoa costulata ovate, of two and a half rapidly en- 

 larging whorls ; the hinder end subangular. Teeth 3 • 1 ■ 3 ; central 

 broad, apex recurved, with five denticulations, the central broadest ; 

 lateral inner broad ; upper inner edge reflexed, toothed ; middle 

 similar, narrower ; outer linear, end reflexed, denticulated. 



The branchial plume of Rissoa reticulata is composed of 12-15 

 single short strands. The mantle has a filamentary process at the 

 angle of the aperture. — Clark, 364. 



The branchial plume of Rissoa parva consists of 15-18 minute 

 vessels attached under and to the mantle and back of the neck. The 

 operculigerous lobe dilates into subcircular lateral wings, bearing 

 close at the junction of the foot and the body the subovate pauci- 

 spiral operculum and a caudal cirrhus. — Clark, 356. 



Head of Rissoa violacea and R. Montagui simple, without ap- 

 pendices, emarginate. Tentacula subulate, nearly as long as the 

 foot. Eyes at the outer base. Foot oblong, truncated in front, 

 slender behind. 



The tentacula of Rissoa proximo are flat, short, smooth, subclavi- 

 form, with long setae. Eyes very large, on the subsemicircular 

 lateral external basal excrescences. Foot large, fleshy, grooved, and 

 longly auriculated in front, and divided behind into two distinct tails. 

 Opercular lobe small, without a caudal cirrhus. Operculum of four 

 to five whorls, the last suddenly enlarged. — Clark, 368. 



Mantle of Rissoa unica is simple, with a filament like Rissoa near 

 the sutural angle. Muzzle slender, rather long. Jaws and lingual 

 riband distinct. Tentacles subulate, obtuse, smooth, with no con- 

 necting tentacular veil or any foldings or apical inflations as in Chem- 

 nitzice. Eyes central, basal. Foot slender, deeply labiated, longly 

 auricled in front. Operculigerous lobe simple, without lateral expan- 

 sion or terminal filament. Operculum ovate, paucispiral. When 

 it walks, the eyes are usually under the margin of the shell. It has 

 no malacological community with Aclis or Chemnitsia, but the muzzle 

 is carried in nearly a similar position as in the latter genus. We 

 must not be misled by the centrality of the eyes at the base of the 

 tentacula, as the vertical cloven disk and corneous jaws, with the 

 simple tentacula, demonstrate that it is merely a Rissoa. Mr. Clark 

 considers Skenea planorbis as a discoidal, and Cerithium reticulatum 

 and Aclis unica as turrited Rissote. — Moll. 373. 



ftf Operculum subcircular, simple ; whorls many, gradually en- 

 larging. Aperture of shell circular, simple. 



3. Skenea. 



Rostrum short, thick, nicked at the end. Tentacles subulate. 

 Eyes basal, exterior. Foot elongate, truncate in front, tapering 

 behind. Opercular mantle with a rounded wing on each side, pro- 

 duced lanceolate behind. — Lovhn. Operculum circular, of many 

 whorls {Adams, Gen. Moll. t. 35. f. 9 a). 



