122 



THE BRITISH NATURALIST. 



[June 



which he captured on April 23rd, at a marshy spot called La Grande Mare, on the 

 west coast of the island. This is the first record of its capture in Guernsey. 



[This Society have issued a most interesting programme of weekly excursions, 

 every Tuesday, and will always be pleased to see visitors. — Ed. B.N.] 



THE GENUS RISSOA (continued). 



BY BROCKTON TOMLIN. 



Sub-genus Alvania, Risso. 



This group was suggested to Risso by the well-known English 

 conchologist Leach in 1826, when the latter was staying in the south 

 of France. Within its limits, as usually understood, is comprised by 

 far the largest proportion of the European species, and also of our 

 British representatives. The usual type of the sub-genus is taken to 

 be Rissoa cimex, L. — -a well-known Mediterranean species but not 

 included in our fauna. The new school of conchologists has not been 

 backward in chopping up so extensive a group as this (there are at 

 least 40 palaearctic species) : it seems to offend their eye to see more 

 than half-a-dozen species in one group. Their vagaries will be noted 

 in dealing with the several species. With Jeffreys we place the 

 following nine British Rissoas in the group Alvania : — striatula, lactea, 

 cancellata, calathus, reticulata, cimicoides, jcffreysi, punctura, and abyssicola. 

 They are well distinguished by the feature of cancellation, generally 

 resembling a beautiful and delicate Lattice-work, which requires in the 

 smaller shells a glass of some power for adequate appreciation. Here 

 also the largest shells of the genus find a place : they are difficult 

 to get alive, but the four first-mentioned, at any rate, can be taken 

 alive in the Channel Isles "between tide-marks. The outer lip is 

 usually strengthened by a rib, and sometimes notched within. We 

 will now turn to the several species in the above-mentioned order. 



Rissoa striatula, Montagu (1803) ought undoubtedly to bear the 

 name of R. carinata, Da Costa. The former name is due to the erroneous 

 identification of this shell with a Einnean species and Da Costa's name 

 has precedence of Montagu's by 24 years. It is an unmistakable 

 shell with three very marked spiral keels, the lattice- work being very 

 fine and indistinct in this species except on the freshest specimens. 

 These keels give the shell its name of carinata (Lat. carina=" keel ")' 

 and are not found in any other species ; hence a special, but hardly 

 necessary sub-genus proposed in 1884 f° r this shell, called Galeodina, 

 Monts. In size it averages 4 to 5 mm., and the colour of a live shell 

 is white, with a slight yellow tinge. The dead shells usually seen in 

 collections, however, are of the purest white. The three keels of this 

 shell are frequenth- very unequal in size, and are accompanied by other 

 smaller ridges. A form occurs with the keels and ridges more equal 



