372 LITTOKINIDJS. 



I conclude this still imperfect monograph by calling on the 

 naturalists of this branch of science to make it more complete, 

 by searching in their respective localities after the animals 

 which continue to elude our view. 



The following Rissoca have not occurred to us alive : 



R. STRIATULA, Montagu. 

 R. striatula, Brit. Moll. hi. p. 73, pi. 79. f. 7, 8. 



R. LACTEA, Michaud. 

 R. lactea, Brit. Moll. iii. p. 76, pi. 79. f. 3, 4. 



R. ZETLANDICA, Montagu. 

 R. Zetlandica, Brit. Moll. iii. p. 78, pi. 80. f. 1, 2. 



R. CRENULATA, Michaud. 

 R. crenulata, Brit. Moll. iii. p. 80, pi. 79. f. 1, 2. 



R. CALATHUS, Forbes and Hauley. 

 R. calathus, Brit. Moll. iii. p. 82, pi. 78. f. 3. 



R. ABYSSICOLA, Forbes. 



R. abyssicola, Brit. Moll. iii. p. 86, pi. 78. f. 1, 2; (animal) pi. J.J. f.3. 

 R. ciNGiLLUS, Montagu. 



R. cingillus, Brit. Moll. iii. p. 122, pi. 79. f. 9, 10 ; (animal) pi. J.J. f.4. 



R. ANATINA, Draparnaud. 

 R. anatina, Brit. Moll. iii. p. 134, pi. 87. f. 3, 4. 



R. VENTROSA, Montagu. 

 R. ventrosa, Brit. Moll. iii. p. 138, pi. 87, f. 1, 5, 6, 7. 



I introduce three new members to this family, the Skenea 

 planorbis of the ' British Mollusca/ and the well-known Ceri- 

 thium reticulatum of all authors. The 'planorbis,' which I 

 have for the second time examined, appears to be absolutely 

 a discoid Rissoa, allowing the necessary margin for specialties 

 of the shape of the foot, operculigerous lobe, tentacula and 

 opercula. These organs greatly vary in the Rissoa, and 

 often differ more with each other, and the type, than even 

 the discoidal 'planorbis.' Ought there not to be two 

 sections in Rissoa, one for the elongated Cerithium reticu- 

 latum, which repeated examinations tell me does not exhibit 

 a difference from it in any material point, and might, as 

 respects the animal, be the type, instead of R. parva, and 



