136 BRITISH MOLLUSKS. 



the remainder have small glassy shells. P. glaber, allied to the 

 last-mentioned, is smooth and very much smaller ; and P. fonta- 

 nus, of a peculiar quoit-like form, convex above, concave below, is 

 smaller still. The last species, P. nitidus, better known to concho- 

 logists as P. lacustris, is distinguished from all others by having 

 the whorls radiately divided internally by septa, and is fairly en- 

 titled to rank as a subgenus (Segmentina) . Sometimes the shell of 

 Planorbis is coiled to the right, sometimes to the left, but in a shell 

 of discoid growth it is not easy to detect the difference. The best 

 mode of observing it, is to notice the obliquity of the aperture. 

 The upper disk of the shell is the side on which the margin of the 

 aperture is the most advanced in growth. The animal of Planorbis, 

 more particularly that of P. corneus, emits a purple fluid, probably 

 blood, when irritated, retiring briskly into its shell. 



Freshwater mollusks, as already observed, are much more evenly 

 distributed over the globe, with much less variation of type, than 

 land mollusks. Planorbis is the least varied of all. The shell is of 

 the same typical form throughout the whole of the Eastern Hemi- 

 sphere, and the species are fewer in number and less varied than in 

 the Western. From Greenland to Tasmania, scarcely more than 

 about thirty have been collected, one-half of which are European, 

 and eleven of these are in Britain. The remainder, of moderate 

 and small size, are scattered in Ceylon, India, Burmah, South 

 Africa, and Australia. We have none from the Malayan Ar- 

 chipelagos. In America, between Chili and the Northern United 

 States, there are not fewer than seventy species of Planorbis illus- 

 trative of two or three quite distinct types. Some of the Bahian 

 and West Indian species are of large size, though not so large as 

 the European P. corneus, and they have a peculiarly compressed re- 

 fulgent shell. The most distinct form of Planorbis is that repre- 

 sented by P. lentus, trivolvis, and bicarinatus, of the United States, 

 in which the whorls are broadly tumid and keeled on both sides, 

 and deeply sunk on both sides in the centre. This, in a country 

 where the Lymncece are of the same type as the British and almost 

 identical in species, is a point to be noted. 



The British species of Planorbis are less local in their distribu- 

 tion than the Helices. Eight out of the eleven are present in all 

 parts of Britain, and on the Continent range southwards into the 

 islands of the Mediterranean, passing, with perhaps two exceptions, 

 into North Africa. P. corneus is found only in our eastern, south- 

 eastern, and midland counties; P. carinatus belongs chiefly to the 



