1911.] NATURAL SCIENCES OF PHILADELPHIA. 161 



the present Mandeville race. But this Mandeville race, as we see it 

 now in the hill-top colonies that remain, is a development from this 

 mingling of the older, high-spired form, and the later hybrid of this 

 with the lower-spired gully form, this hybrid having come into the 

 Mandeville district as a second migration wave. And the result of 

 this mingling here to the south of Somerset has become, on account 

 of its isolation in the hill-top colonies, a smaller, still higher-spired 

 race than the ancestral one. 



Causes of the Variation. 



The rise of the spire from the bottom to the top of the hill or from 

 the valley or gully to the top of the hill has been noted in other cases, 

 but the cause assigned for the change of form has generally been the 

 hypsometric change of pressure. Thus Arnauld Locard, in his 

 Etudes sur les variations malacologique du Bassin du Rhone, published 

 in 1881, says that the elongation of the spire is one of the effects of 

 change of altitude upon molluscs. This had been noted previously 

 by A. C. Recluz, in the case of Helix pomatia in the mountains of the 

 Auvergne, and also in Helix aspersa in the region of the Midi. Locard 

 made similar observations in the case of H. pomatia in passing from 

 Lyons to Grenoble. Both observers noted a rise in the spire in passing 

 from the lower to the higher altitude and both attribute this change 

 in form to the diminution of air pressure in the higher altitude. In a 

 paper published in 1904 3 Raffaelo Bellini reiterates this view that the 

 elongation or rise of the spire is to be attributed to the influence of the 

 diminished atmospheric pressure. Bellini's observations were on 

 species observed on the island of Capri, and are embodied in a previous 

 paper entitled, Alcune osservazioni sulla distribuzione ipsometrica 

 dei molluschi terrestri neW isola di Capri, 4 " in which he notes the elonga- 

 tion of the spire in passing from the base to the tops of the hills, and 

 attributes it to the diminution of the atmospheric pressure. That 

 this is not the predominant cause in the case of the species considered 

 in this paper the foregoing descriptions will make clear, for here we 

 find at Somerset, some 2,250 feet in elevation, the lowest-spired forms 

 of P. a. goniasmos encountered, while at the Somerset Road colony the 

 spire is higher, but the altitude above sea level lower, less than 2,000 

 feet. At the Ridge near Lincoln the elevation is about 2,800 feet, 

 but the form of P. a. goniasmos found here is lower in spire than that 



3 R. Bellini, L'influenza dei mezzi come causa di variazione e di disperzione 

 nei molluschi, Bolletino del Societa da naturalisti in Napoli, XVIII, 1904, p. 159. 



4 Rendiconto del II Cong. Zool. Italia.no, Naples, 1901. 



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