46 SUDDEN APPEARANCE AND DISAPPEARANCE chap. 



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

 average sized snail of moderate pace progresses at the rate of 

 about a mile in IG 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 moUuscan life, Mollusca 

 are certain to be found in it sooner or later. " Some 2 3 years 

 ago," writes Mr. AV. Nelson,"-^ " I was in the habit of collecting 

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

 time the only moUuscan forms found there were a dwarf form of 

 Sphaerium lacustre, Pisidium pusillum, Planorhis nautileus, 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 PL 

 carinatus, Limnaea stagnalis, and Ancylus lacustris turned up ; 

 and during June, PI. contortus was found in this small but prolific 

 pond." Limnaea glutinosa is prominent for these remarkable 

 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 hand- 

 fuls. After that year they did not appear numerous, and after 

 three or foiu: seasons they gradually disappeared.^ Physa 

 {Aplecta) hypnorum is noted in a similar way. In February 1852, 

 for instance, after a wet month, the water stood in small puddles 

 about 3 feet by 2 in a particidar part of Bottisham Park 

 which was sometimes a little swampy, though usually quite dry. 

 One of these puddles was found to contain immense numbers of 

 the Aplecta, which up to that time had not been noted as occm-- 

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

 disappeared and was never again noticed in the locality.^ 



^ Thomas, quoted by Jeftreys, Brit. Conch, i. p. oO. - Journ. of Conch, iv. ji. 1 17. 



■^ Rev. L. Jenyns, Ohscrvations in Nat. Hist., p. 318. ■* Id. ib. p. 319. 



* Further detailed examples will be found in Kew, The Dispersal of Shells, jip. 5-26. 



