442 



MOLLUSC A OF SHALLOW SEAS. 



These genera are found in the Secondary Rocks in the Purbeck 

 and Wealden Beds ; and in the Lower Tertiary lacustrine beds of the 

 Me of Wight, known as Headon and Bembridge Beds; and in the 

 valley gravels. 



Fresh- water deposits often contain pulmonate land gasteropoda, 

 such as Helix, Bulimus, Pupa, Clausilia, Cyclostoma ; together with 

 land mammals, birds, reptiles, and laud plants. 



The absence of fossil species of the following groups 

 is to be expected in fresh-water strata : Foraminifera, 

 Sponges, Corals, Echinoderms, Cephalopod Mollusca, 

 Brachiopoda, and Bryozoa. All these groups are typi- 

 Fig. 85. Helix, cally marine ; though foraminifera live in the Thames 

 at Gravesend, and Spongilla is characteristic of fresh- 

 water ponds. 



Estuarine and Brackish- water Deposits. There are some genera 

 which are typical of the mouths of rivers, and such forms may be 

 termed estuarine ; but whether they have always existed under such 

 conditions, may well be doubted. Among the more re- 

 markable of such genera may be mentioned Potamides, 

 Cerithium, Melania, Melanopsis, Auricula, Nerita, Mya, 

 Scrobicularia, Cyrena ; and a predominance of such types 

 in a stratum would justify us in affirming its estuarine 

 origin. Examples of strata showing such conditions may 

 be seen in the Woolwich and Reading beds between Upnor 

 and Dulwich, and certain beds of the fresh- water tertiary 

 series of the Isle of Wight. But towards the mouths of 

 rivers the marine shells become modified in form and 

 regularity of growth and ornament, by the influence of 

 fresh water, so that varieties are developed which, in 

 their divergence, at first suggest to the collector a multitude- 

 Such variation is well known in the Caspian, and Sea of 

 Aral, and Black Sea, at the mouths of rivers, especially among species 

 of Cardium. It is strikingly shown in the estuarine part of the Red 

 Crag, known as the Norwich Crag, in which the Fusus antiquus and 

 Purpura lapillis are singularly variable. 



Shallow-water Deposits. The geologist will always judge from 

 physical evidence of the conditions of deposition of a stratum. Con- 

 glomerates and grits must be shallow-water deposits. Sandstones are 

 likely to be accumulated in shallow water, and no evidence can over- 

 ride the presence in them of false bedding and ripple-mark. But 

 mineral character alone is not always proof of nearness to shore of a 

 stratum, for that may change with the nature of the coast wasted. 

 At the present day no one would hesitate to affirm low-water condi- 

 tions for a collection of living British shells which included 



of species. 



Purpura. 



Patella. 



Cardium. 



Nassa. 



Haliotis. 



Tapes. 



Trochus. 



Pecten. 



Solen. 



Littorina. 



Mytilus. 



Pholas, 



