RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 349 



All living forms of the Melaniidse are regarded as freshwater niollusks, 

 but a few species appear to be able to live in waters which arc in some 

 degree saline. Certain fossil members of this family have been found 

 in such association with other lnollusean remains as to indicate that 

 they were capable of living in saline waters. 



Much the greater part of the Cerithiidse inhabit marine waters, but 

 some species are known to find a congenial habitat in brackish waters, 

 and a few are known to range into adjacent fresh waters. 



As a rule, the Neritidaeare found in either marine or brackish waters. 

 but a few species are known to live in fresh water. 



The Trochus-like shells which have been found in Lake Tanganyika 

 probably do not belong to the Trochidae. The trochids are therefore 

 regarded as distinctly marine. 



A few of the Acmaud limpets found in Borneo are reported to pass 

 from saline waters into fresh. The Acuueidae are not uncommon in 

 brackish waters, but members of the family are most abundant in ma- 

 rine waters. 



The Otinidae, Auriculuhe, Siphonariidae, and Gadinidse, are air- 

 breathing moliusks living upon the margins of both marine and brack- 

 ish waters. The Amphibolidae also usually inhabit the sea margin, but 

 some of them appear to find waters of less than marine saltness not 

 uncongenial. 



Although the Bullidae are, as a rule, strictly marine, two genera, 

 namely, Haminea and Tornatella, have been found in the mud of brack- 

 ish water lagoons. 



As indicated in the paragraph preceding this table, all the members 

 of the (lasteropod order Nudibranchiata, together with all those of the 

 Pteropod order Gymnosomata are omitted from it because none of the 

 species possess more than a minute embryonic shell, and therefore no 

 identifiable fossil remains of any members of these orders are likely to 

 be discovered. All the Tunicata and land Pulmonata also arc omitted, 

 the former because the character of the body is always such that re- 

 mains of it are not likely to be found fossilized, and the latter, because 

 they have no direct relation to an aquatic habitat. 



Of the twelve families mentioned in the foregoing notes as having 

 representatives in more than one of the three kinds of habitat indicated 

 in the table, fully one-half of them are so generally found only in marine 

 waters that geologists usually are inclined, in the absence of contrary 

 evidence, to regard fossil representatives of them as indicating a marine 

 origin for the strata in which they may occur. Three of the other fam- 

 ilies are so generally found only in fresh waters that the converse of 

 the foregoing remarks would apply to them. Representatives of the 

 others are so often found in both marine and nonmarine waters that 

 in the case of fossil representatives it is always necessary to have cor 

 roborative evidence as to the probable character of the water in which 

 they lived. 



