352 REPORT OF NATIONAL MUSEUM, 1892/ 



fresh waters equally congenial, and some species appear to range into 

 both brackish and fresh water. This family, however, is represented 

 by a greater number of species in uonmarine than in marine waters. 



The Adacnidae are seldom or never found in waters of full marine 

 saltness, but they are found in such waters as those of the fresher por- 

 tions of the Black sea, in estuarine waters, and in those which are quite 

 fresh. Therefore fossil members of this family are regarded as indicat- 

 ing a nonmarine origin for the deposits containing them. 



One feels so fully warranted in regarding the living Arcidse ;is a 

 marine family that he expects to find no exception among its fossil 

 members, but the case of the living fresh-water Stvtphiila gangetica 

 shows that this rule is not absolute. Therefore it is possible, if not 

 probable, that fossil members of this family may be discovered in non- 

 marine formations. 



While the Mytilida* all thrive in waters of full marine saltness, some 

 of them also thrive in brackish waters. Entirely fresh waters, however, 

 seem to be uncongenial to all of them. 



All the Ostreidse thrive in waters of full marine saltness. They thrive 

 equally well in the less saline waters of estuaries and bays, but never 

 in entirely fresh waters. Therefore the presence of fossil representa- 

 tives of this family in any formation is taken to indicate that the water 

 in which it was deposited was at least in some degree saline. 



The Anomiidie are far more abundant in marine than in any other 

 waters, but they are frequently found in the less saline waters of bays 

 and lagoons. They are not known to occur in fresh waters, and it is 

 therefore inferred that no fossil members of this family had a fresh- 

 water habitat. 



Brachiopoda. — Every Known member of all the families of this class 

 is exclusively marine. Therefore geologists regard the presence in 

 any formation of Brachiopod remains as sufficient evidence of its 

 marine origin, in case there is no indication of its accidental presence. 



Poh/zoa. — With the exception of the three fresh- water families, Pec 

 tinatellidse, Cristatellkhe, and Plumatellidse, all the Polyzoa inhabit 

 saline waters, most of them living in those of full' marine saltness. A 

 few of them range into brackish waters, but as a class they are so gen- 

 erally marine in their habits that the geologist rarely hesitates to con 

 elude that any formation is of marine origin which contains fossil 

 polyzoan remains. This conclusion is supported by the fact that the 

 living fresh-water representatives of the class are rarely provided with 

 skeletal, protective, or supporting parts which resist decomposition 

 after the death of the colonies of minute animals. 



ANNCLOIpA. 



The Annuloida are divisible into two classes, the Seolecida and Ech- 

 inodermata. All the Seolecida are omitted from this review because 

 the remains of no representative of any of its families is likely to be 

 discovered in a fossil state. 



