Operculum of Georissa. 177 



and operculum, to believe that either Kiister or I had com- 

 mitted some oversight in the examination of the animals, and 

 that they are in reality alike. But the circumstance that 

 Hydrocena is a truly marine species, living in water, whilst 

 all the species of Georissa are found on hills at a distance from 

 the sea*, renders it probable that a difference really exists ; 

 and the characters of the lingual ribbon tend to bear out that 

 distinction. 



The lingual teeth of Hydrocena have been figured by Tros- 

 chel in the ' Gebiss der Schnecken,' vol. i. Taf. 6, and described 

 at page 83. They differ from those of Georissa in the charac- 

 ters of the central teeth, which, however, are rudimentary in 

 both forms, and have not been clearly made out in Georissa. 



Troschel regards the genus Hydrocena as forming the type 

 of a family of Mollusca with affinities to the Helicinidas and 

 the Neritinidaj — a view which appears best to meet the circum- 

 stances of the case. If, therefore, the genus Georissa, as a 

 land-shell, be kept distinct from Hydrocena, it will form a 

 second genus of the family. 



But I cannot conclude without calling attention to the sur- 

 prising resemblance shown in this case by a true land-mollusk 

 to an undoubted marine form, as one more addition to the 

 numerous arguments against separating the Cyclostomidse, 

 Cyclophoridai, and Helicinidas from their natural allies living 

 in fresh or salt water. 



Note on Hydrocena tersa, Benson, and H. milium, Bens. 



Two minute shells were described by Mr. Benson in the 

 i Annals ' for 1853 (ser. 2. vol. xi. p. 285), under the names of 

 Cyclostoma tersum and G. milium. They were found in moss 

 brought from the Khasi Hills. Subsequently, in 1856 (oj). cit. 

 vol. xvii. p. 232), Mr. Benson referred both species, together 

 with C. sarritum, to the genus Hydrocena. When, in 1864, 

 I proposed the genus Georissa for the last-named species and 

 its allies, I suggested that C. tersum and C. milium, which I 

 had never seen, might perhaps belong to it. Neither the ani- 

 mals nor opercula of these two species were known to Mr. 

 Benson, nor have they hitherto been described. 



I am indebted to Capt. Godwin- Austen for specimens of a 

 shell which I have no hesitation in referring to Mr. Benson's 

 Cyclostoma tersum, and for figures of the animal, operculum, 

 and lingual ribbon. The original specimen was probably 

 weathered; when fresh, the shell is of the colour of horn. 



* G. sarrita is found at a height of 4000 feet above the sea, on the 

 Khasi Hills. 



