414 PLIOCENE MOLLUSC A. 



dwarfed in their growth by the quantity of fresh water poured by rivers into 

 that inland sea; 1 Prof. Ed. Forbes also, describing a visit to a brackish lake in 

 South Arran, remarks that he found in it an interesting variety of Gardium edule, 

 the shells of which were remarkably thin and brittle. 2 



Such facts suggest that there may have been, during the later stages of the 

 Crag period, a gradual diminution in the salinity of the Crag sea. This view would 

 also explain the comparative and increasing poverty of the molluscan fauna of the 

 Icenian deposits as compared with its great variety and abundance during those of 

 the periods preceding. 



It seems that during an early stage of the Red Crag history the sea was closed 

 to the south and open to the north. 3 The advance of the great ice-sheet to which, 

 in the opinion of most geologists, the distribution of Scandinavian erratics over 

 northern Europe was due, may have gradually obstructed this northern communi- 

 cation and eventually have brought it to an end. Volumes of fresh water from the 

 south, however, under such circumstances, especially during summer, would have 

 brought into existence a lake, the water of which became increasingly brackish. 

 Some such cause as this may have changed the character of the Pliocene molluscan 

 fauna of the North Sea and finally have exterminated a great part of it. 

 Although communication with the ocean was subsequently re-established by the 

 catting of the Straits of Dover, at first perhaps an overflow channel to the Icenian 

 lake, the molluscan fauna of the East Anglian region has never regained the 

 richness and variety it possessed during the earlier stages of the Pliocene epoch. 4 



Genus BITTIUM (Leach), Gray, 1847. 

 Bittium reticulatum (Da Costa). Plate XLI, figs. 1 — 3. 



1778. Strombiformis reticulata*. Da Costa, Brit. Conch., p. 117, pi. viii, fig. 13. 



1833. Cerithium reticulatum, S. Woodward, G-eol. Norfolk, p. 36, pi. i, fig. 2. 



1853. Ceritkium reticulatum, Forbes aud Hudey, Brit. Moll, vol. iii, p. 192, pi. xci,-figs. 1, 2. 



1863-69. Cerithium reticulatum, Jeffreys, Eep. Brit, Assoc. (Newcastle-on-Tyiie), p. 77, 1863; Brit. 



Conch., vol. iv, p. 258. 1867; vol. v, p. 217, pi. lxxx, fig. 4, 1869. 

 1870-1915. Cerithium reticulatum, A. Bell, Journ. de Conch., vol. xviii, p. 350: Ann. Mag. Nat. 



Hist. [1], vol. vi, p. 215, 1915. 

 1872. Cerithium reticulatum, S. V. "Wood, Mod. Crag Moll., 1st Suppl., pt. i, p. 50, pi. v, fig. 22. 



1 Antiquity of Man, 3rd ed., p. 13. 



2 Nat, Hist. Brit, Seas, p. 230, 1859. 



3 See Quart. Journ. Geol. Soc, vol. Iii, p. 754, 1896. 



1 It should be stated, however, that the observations of Prof. Batesou on the variations of C. edule 

 in certain lake-basins iu Central Asia which seem to have been coincident with changes in their 

 salinity (Phil. Trans. Roy. Soc. London, vol. clxxx, p. 316) do not correspond with those of Dr. 

 Munthe. In any case, however, the influence of salinity on the growth of mollusca must have been 

 indirect, probably in affecting the supply of suitable food. 



