196 



HELIX PIS AN A IN PORTO SANTO. 



By Professor T. D. A. Cockerell. 



(Communicated by B. B. Woodward, F.L.S.) 



Read 13th May, 1921. 



No one knows when or how Helix pisana first reached the Madeira 

 Islands, but it abounds in Porto Santo and the adjacent islets, 

 and equally in the vicinity of Canipal, on the Island of Madeira. 

 Possibly it was brought by the Moors, who used to frequent these 

 coasts, or it may have come through some " natural " agency, 

 at present unexplained. It is not found in the Pleistocene deposits, 

 although old shells get mixed with the fossils, and have been 

 erroneously reported as such. The presence of a great number of 

 species of endemic snails has in no wise hindered its multiplication, 

 or spread. In the vicinity of Canigal, H. pisana is represented by 

 a small race. On Porto Santo and the islets nearby it presents many 

 varieties, none of which seems to occur at any point to the exclusion 

 of all others, though various localities are noteworthy for shells of 

 a certain type. There are, however, two general types of pisana, 

 which, though occurring intermixed, appear to retain their characters 

 and possibly do not interbreed. One is the typical, thinnish form, 

 usually conspicuously banded ; the other is thick, opaque white, 

 with a rosy aperture. The latter was actually described from Porto 

 Santo as a new species. Helix calcarea, Pfeiffer. The type is in the 

 British Museum. It does not depend on environmental conditions ; 

 thus it occurs with typical pisana on the small islet called Censuras, 

 off Porto Santo. These Censuras " calcarea " shells are small — 

 max. diam. 13-14: mm., alt. 9-10 mm. ; the extreme apex is 

 reddened. On the Ilheo de Baixo a very extraordinary shell (mut. 

 grandis) of the calcarea type was found by Miss Nancy Paterson. 

 It is thick, white, with rosy mouth ; 6j whorls, spire elevated ; 

 diam., max. 24, min. 22, alt. 23 mm. ; umbilicus to apex 15'5 mm. 

 Gigantism in plants has been found to be correlated with a change 

 in the number of chromosomes, but in animals it may be due to 

 changes in the internal secretions. Among snails it seems to occur 

 especially in certain places and at certain times. Thus the H. 

 nemoralis I found in Portugal were unusually large ; Helix ox Leptaxis 

 groviana from the Pleistocene at Canipal is much larger than the form 

 of the species now common in Madeira. Thus it seems possible that 

 environmental conditions may, at least in part, control gigantism 

 is snails ; though the large pisana from I. de Baixo seems to have 

 arisen, without any external cause, in a normal population of the 

 calcarea type. The name calcarea is preoccupied, but similar shells 

 have been described from the continent of Europe. In form and 

 colour the large I. de Baixo pisana is very like rkadanica Locard, 

 from Oporto. The H. subpisana Bourg., from Tunis, Spain, and the 



