LITTORINA r.TTTOREA. 047 



Fossil specimens of Littorina littorea departing widely and in various directions 

 both in form and sculpture from the typical shell, as will be seen by some of those 

 figured on Plate LIl, have been found locally in the Icenian Crag, and especially at 

 Thorpe, Postwick and Bramerton near Norwich. By Samuel Woodward, Avho 

 described some of them in 1833, they were regarded as specifically distinct, but in 

 1848 S. V. Wood grouped them all as varieties of the present species. In 1872, 

 however, in the 1st Supplement (pt. i, p. 79), he suggested that some of them might 

 be varietal forms of an allied but different species, L. rudis, as to which, however, 

 Jeffreys says that L. littorea may be known by being usually twice its size, having a 

 much slighter suture, a more elongated and sharply pointed spire, and a straight 

 upper lip. 



Assuming that the many widely differing shells referred to may represent 

 varietal forms of L. littorea, locally developed at a certain stage of the Crag history, 

 it may be asked why did not some of them succeed in establishing themselves as 

 distinct species. So far as I know, only one or two have been recorded from our 

 Pleistocene deposits, or as now living in British seas. 



In Vol. i, p. 413, of this Monograph I have attempted to explain the increasing 

 poverty and the decreasing vigour of the molluscan fauna of the Crag Sea during 

 the Icenian period^ by the view that the advance of the glacial ice cut off the com- 

 munication of that region with northern seas while fresh water continued to pour 

 into it from the south. Such conditions would have diminished the salinity of the 

 water, gradually changing the marine conditions of the Red Crag first into a 

 brackish and eventually into a fresh-water lake in which marine mollusca could 

 not have existed ; these anomalous Littorinas having been only locally developed 

 could not have been reintroduced when the ice disappeared. Whether this 

 hypothesis be accepted or not the introduction and subsequent extinction of such 

 shells is interesting and deserves the attention of conchologists. 



Var. vulgaris, S. V. Wood. Plate LII, figs. 9—11. 



1814. Turbo littoreus, J. Sowerby, Miii. Couch., vol. i, p. 163, pi. Ixxi, fig. 1. 



1848. Littorina littorea, var. vulgaris, S. V. Wood, Moii. Crag Moll., pt. i, p. 118, pi. x, fig. 14. 



1871. Littorina littorea, Morcli, Geol. Mag. [1], vol. viii, p. 396. 



Varietal Characters. — Differs from the typical form of British seas in its 

 smaller size, and its distant and well-marked coloured bands, the sculpture 

 approaching that of the var. nigrolineata of L. rudis. 



' At the Norwich stage of the leeniau Crag the species of mollusca were less numerous aud 

 generally of a more fragile character than those of the Eed Crag, while during the Weybourne period 

 they numbered not more than about 60 altogether, most of the specimens being the shells of a 

 small bivalve, Tellina balthica. 



