GEOLOGICAL DISTRIBUTION OF GASTEROPODA. 393 



guidance, in determining the circumstances under which a par- 

 ticular deposit was formed. But he cannot be sure in regard 

 to this, from the examination of one or two shells only ; since 

 there are many genera, which contain species of both kinds. He 

 is guided, therefore, by the comparison of all the shells contain- 

 ed in the deposit, with their nearest allies amongst those now 

 existing. Sometimes there is such a mixture of marine and 

 fresh-water shells, as to induce the belief, that the deposit was 

 formed in the estuary at the mouth of a river, of which both 

 might be inhabitants at once. In other cases, the shells are so 

 exclusively fresh-water, as to indicate that the deposit was 

 formed at the bottom of a river or lake ; and in this case, as 

 might have been expected, it is usually of no great extent. 

 When the nature of the shells indicates a deposit from the 

 bottom of the sea, the same shells are frequently found in strata, 

 which differ greatly in their mineral materials, and which present 

 themselves at very different parts of the earth's surface ; and 

 they thus afford important assistance to the Geologist, in deter- 

 mining the real correspondence between these deposits. It is a 

 curious fact, that in all the earlier rocks, down to the chalk- 

 formation, the remains of the carnivorous Gasteropods bear a 

 very small proportion to those of the herbivorous group ; and their 

 place would seem to have been then supplied by the numerous 

 Cephalopods of predaceous habits which then infested the seas 

 ( 968). Nearly all of these disappeared after the Chalk was 

 formed ; and the proportion of the carnivorous Gasteropods ex- 

 hibits a remarkable increase from that period. 



