RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 351 



ORDER METAKKIIIl'T.K. 



Murine 



Tridacnidse 



Brackish. 



Fresh. 



ORDER HETER< >M V ARIA. 



Mytilidje Mytilidae. 



ORDER MONOMYARIA. 



Pinnidse 



Pteriidae — 

 Vulsellidae . 

 Spondylidse . 



Lhuida; 



Pectinidse. . . 

 Placunidse . . 

 Auomiidse . . 

 Ostreidiv 



Anomiidas. 

 Ostreidaa . . 



The following remarks are confined to those families which are known 

 to have representatives in more than one of the three kinds of aquatic 

 habitat indicated in the foregoing table. 



The Teredinida 1 are properly regarded as a marine family, but at 

 least one living species is known in a fresh-water habitat, namely, in 

 the lower Ganges. Therefore the discovery of a fossil member of this 

 family is not of itself positive, although presumptive, proof ot the marine 

 origin of the formation containing it. 



A similar remark may be made concerning the Solecurtidae, a few- 

 species of which have been found in brackish waters, and in rare cases 

 some have been known to range into fresh waters. 



The Corbulid;e are common in bothjnarine and brackish waters, and 

 they are occasionally found to have passed into adjacent fresh waters 

 or to have survived the freshening of the saliue waters in which they 

 formerly lived. Fossil members of this family are often found with 

 only marine associates, quite as often with brackish water associates, 

 and in rare cases all associated species are fresh-water forms. 



Some species of both the Psammobiidse and Glauconomkke have 

 been found in estuarine waters of considerably less than marine saltness. 

 Therefore fossil species referable to either of these families are likely 

 to be found associated with an estuarine fauna. 



The Cyrenidie like the Corbulidae, are represented in marine, brack- 

 ish, and purely fresh waters, and the paleontological significance of both 

 families is similar. 



The range of habitat of the Cyrenoididae is similar to that of the 

 Cyrenidae, except that none of them are known to inhabit purely tresh 

 waters. 



The Dreissenida? as a family appear to find marine, brackish, and 



