PECTEN. 57 



blended in specimens of P. varius and what I consider 

 to be its two principal varieties. Some are broader and 

 flatter than others : the number of ribs varies from 27 

 to 45, and consequently in their comparative fineness ; 

 and the white of P. niveus is sometimes beautifully 

 tinted with purple, and passes into different shades of 

 other colours. I have taken P. niveus only in every 

 part of the Hebrides, and I have never seen a single 

 specimen of P. varius from that district. In Loch Fyne 

 and at Jura an intermediate variety occurs. A speci- 

 men from the former locality has 36 ribs, and one from 

 the latter 32 ribs. I noticed in the collection of M. 

 Martin, at Martigues, a white variety having also 32 

 ribs. A still more puzzling form was sent to me in 

 1852 by Mr. Barlee from Glengariff in Bantry Bay, 

 which clearly connects the two species ; and the variety 

 purpurea forms another link in the chain of specific 

 identity. I believe this varietal difference arises from 

 habitat. The strong and few-ribbed P. varius lives on 

 oyster-banks and rough ground on an exposed coast; 

 while the delicate and many-ribbed P. niveus is only 

 found in sheltered lochs and arms of the sea, moored by 

 its strong byssus to the upper surface of the broad and 

 smooth fronds of Laminarice. The very circumstance of 

 the latter being confined to a limited district is suspi- 

 cious as regards its specific distinction. Dr. Gray, in com- 

 menting on the species in question (Ann. Phil. no. 59. 

 p. 387), says, " Mr. Macgillivray only compares it with 

 P. varius, perhaps not aware that Pecten Islandicus, 

 Lam., of which this shell appears to be only a variety, has 

 long been known as a British species." The last-named 

 species, however, has never been found in Great Britain 

 except as an upper tertiary fossil ; and it has only a 

 generic resemblance to Macgillivray 's shell. 



D5 



