^2 



THE BRITISH NATURALIST. 



[April 



producing mutations not noticed in Europe. This may be set down 

 to the disturbing influence of new environment, but it remains to be 

 seen whether after many years the variation will be kept up, or a new 

 race will be formed, or whether, as is perhaps most likely, the species 

 will tend to revert to the normal. This suggests experiments as to 

 the inheritance of acquired characters, which may well be undertaken 

 with snails. In the case of Paludina, recorded by Mr. Gain, it appears 

 that the large size, which may have been due to favourable 

 environment, was not inherited. — T. D. A. Cockerell, f.z.s., f.e.s., 

 Institute of Jamaica, Kingston, Jamaica, Febmary 21st, 1892. 



The Genus Rissoa.— i?^s50<7 is one of the smallest genera as 

 regards size, and one of the largest, if Vv^e reckon the number of 

 species, that inhabit British vv'aters. The former consideration makes 

 these pretty little mites a real stumbling block to the Conchologist at 

 starting, and he is apt to give them up as a bad job when he finds 

 that we boast of 25 species. It is, however, quite possible to quickly 

 acquire a certain facility in "spotting"' several species, and by the 

 process of elimination one gradually is able to work out almost all the 

 commoner Rissoas, temporarily disregarding at least half-a-dozen, 

 whose rarity takes them out of the ken of the ordinary collector. The 

 variations in the species themselves are not numerous, being chiefly 

 confined to the protean R. parva, one of the commonest of small shells, 

 which is frequently referred to me for determination under various 

 queried names and with every variation of sculpture, from the smooth 

 shell (var. iiiternipta , Adams) to those with the strongest ribs ; the 

 colour too of R. parva is variable, and a dark chocolate shell 

 occasionally occurs which Canon Norman calls var. nigra. Created 

 by "'Freminville in 1813, and named after the old Nice Naturalist. 

 Risso, tiie origin of the genus can be traced back at least into Eocene 

 times in the Paris Basin, and many recent species actually occur also 

 in the Pliocene and Pleistocene of Italy, Sicily, England, Rhodes, etc. 

 The range of the genus is universal, though principally in temperate 

 waters, and the habitat extends from between tidemarks to great 

 depths— R. abyssicola, Fbs. came up, off" Malta, from 350 fathoms, 

 this being, as its name implies, a deep-Vv^ater species, and no doubt 

 the "Challenger" Reports furnish much greater depths even than 

 this. It is, however, a littoral genus in the majority of cases, and Dr. 

 Jeffreys divides our list into 15 littoral and 10 coralline and deep- 

 water species. Woodward gives the total number of species in the 

 genus as about 70, but Dr. Kobelt catalogues no European species only 

 in his " Pzodrornus 1888, and Canon Norman in the same year 

 publislied a carefully rex ised catalogue of his shells, listing no less 



* Journal des Mines, 1813. Bull, de la Soc. Philoraatique de Paris, 1814 (Desmarest). 



