FRESII-WATER SHELLS. 23 



in one instance, a specimen of Planorhis marginatus 

 [= PL complanatiis\ presumably dead, and living fish 

 are said to have come up from a depth of 175 feet,^ but 

 I am assured that no such underground transportal to 

 Trafalgar Square is likely to have taken place. Mr. 

 Layard has suggested that the LimncecE were brought 

 to the basins from the ornamental water in one of the 

 parks, " attached to the feathers of the sparrows who 

 bathed, first in one and then in the other." - When the 

 fountains cease to play, as I was told by a policeman 

 on duty in the square, many birds— both sparrows and 

 pigeons— bathe in the small upper basins from which 

 water falls into the large basins in which the Limncece 

 are seen. It seems just possible, however, as Mr. Reid 

 has remarked to me, that " some misguided naturalist 

 with a mania for * acclimatization ' may have placed 

 some snails in the basins, or one of the boys who will 

 try to catch fish there may have brought his line 

 straight from the park with the small snails or eggs 

 attached." 



Aquatic molluscs, as is well known, commonly live 

 in marshes and swamps which are dry during a great 

 part of the year, and they occur also in other non- 

 permanent water, in that collected in hollows after 

 rains, in roadside puddles, &c. When the water is 

 derived from the overflowing of rivers the presence of 

 the animals is easily understood, for they are known to 

 be carried from place to place with flood-water ; their 



^ See " Lyell's Principles," ed. 12, i. (1875), 39o-i- 

 2 E. L. Layard, " Nature," xxxviii. (1888), 296. 



