VI.] INTRODUCTION. CV 



which has the effect of casting on the shore a collection 

 of spolia marina, dislodged by the ground-swell from 

 considerable depths^ as well as of many land and fresh- 

 water shells, which have been washed down by the river 

 or stream and thus become mixed with those from the 

 sea. This phenomenon frequently occurs in some of our 

 upper tertiary and more recent deposits, and shows the 

 regularity with which such physical operations have been 

 repeated dui'ing periods of the duration of which we can 

 form no conception. In the same zone are also com- 

 prised estuaries, which form deep but narrow indenta- 

 tions of the sea- coast, and are the channels, as well as 

 the outlet, of tidal rivers. The water of these estuaries 

 is always more or less brackish. They are inhabited by 

 peculiar MoUusca, viz. Assiminia Grayana, Melampus 

 myosotis, and the several species of Hydrobia above men- 

 tioned. The few pelagic mollusks which occasionally, but 

 unwillingly, \4sit our seas, are also met with in the littoral 

 zone, being cast on shore generally after a continuance 

 of westerly gales. These consist of species oi lanthina, 

 Spirula Peronii, and a few Pteropods, some of which 

 have but a doubtful claim to be considered indigenous 

 productions of our seas. The present zone has been 

 subdivided by Forbes and Hanley into four intermediate 

 lines or strips, each of which is said to be inhabited by 

 its own peculiar set of Mollusca ; but the great variety 

 exhibited by our seaboard, as well as its geological for- 

 mation, seems scarcely to warrant such a subdivision. 

 For instance, Trochus umbilicatus and T. lineatus, which 

 are assigned by these authors to the fourth or lower- 

 most line, in which they usually occur on our western 

 and south-western coasts, inhabit the second line on the 

 shores of the Bristol Channel, as well as those of the 

 north-west of Ireland. 



