4.6 SUDDEN APPEARANCE AND DISAPPEARANCE chap. 



ground on which it travels. It has been calculated that an 

 averaged sized snail of moderate pace progresses at the rate of 

 about a mile in 16 days 14 hours. ^ 



Sudden Appearance of Mollusca. — It is very remarkable 

 to notice how suddenly Pulmonata seem to appear in certain 

 districts where they have not been noticed before. This sudden 

 appearance is more common in the case of fresh-water than of 

 land Mollusca, and there can be little doubt that, wherever a 

 new pond happens to be formed, unless there is something in its 

 situation or nature which is absolutely hostile to molluscan life, 

 Mollusca are certain to be found in it sooner or later. " Some 

 23 years ago," writes Mr. W. Nelson,^ " I was in the habit of 

 collecting shells in a small pond near to the Black Hills, Leeds. 

 At that time the only niolluscan forms found there were a dwarf 

 form of Sphaerium lacustre^ Pisidium pusillum., Planorhis nau- 

 tileus, and Limnaea peregra. About 10 years ago I resumed my 

 visits to the locality, and found, in addition to the species already 

 enumerated, Planorhis corneus. These were the only species 

 found there until this spring [1883], when, during one of my 

 frequent visits, I was surprised to find Physa fontinalis and 

 Planorhis vortex were added to the growing list of species. Later 

 on PI. carinatus, Limnaea stagnalis, and Ancylus lacustris turned 

 up ; and during June, PI. eontortus was found in this small but 

 prolific pond." Limnaea glutinosa is prominent for these re- 

 markable appearances and disappearances. In 1822 this species 

 suddenly appeared in some small gravel pits at Bottisham, 

 Cambs., in such numbers that they might have been scooped 

 out by handfuls. After that year they did not appear numer- 

 ous, and after three or four seasons they gradually disappeared.^ 

 Physa QAplecta^ Jiypnorum is noted in a similar way. In Feb- 

 ruary 1852, for instance, after a wet month, the water stood in 

 small puddles about 3 feet by 2 in a particular part of Bottisham 

 Park which was sometimes a little swampy, though usually quite 

 dry. One of these puddles was found to contain immense num- 

 bers of the Aplecta, which up to that time had not been noted as 

 occurring in Cambridgeshire at all.* In a few days the species 

 entirely disappeared and was never again noticed in the locality.^ 



1 Thomas, quoted by Jeffreys, Brit. Conch, i. p. 30. ^ Journ. of Conch, iv. p. 117. 



" Rev. L. Jenyns, Observations in Nat. Hist. p. 318. ^ Id. ib. p. 319. 



^ Further detaihid examples will be found in Kew, The dispersal of Shells, pp. 5-26, 



