34S EtEPORT OP NATIONAL MUSEUM, 1892. 



Pteropoda. 

 ORDER THECOSOMATA. 



Limacinkla3. 

 HyalicUe .... 

 ( !j nil.iiliid.i- 



Brackish. 



Fresh. 



ProsocephaJ-a. 

 ORDER SOLENOCONVU.K. 



Deiitiliid.i 



The foregoing table of the families of living Gasteropoda is, as 

 already stated-, constructed upon the same plan as that of the fishes, an 

 explanation of which plan and its significance is given on page 33l>. 

 The list includes the names of only those families at least some mem- 

 bers of which possess shelly or skeletal parts that are likely to resist 

 decomposition after the death ot the animal. Other families are not 

 included, because it is only by means of those hard parts that the liv- 

 ing can be compared with fossil forms. The significance of placing the 

 name of a family in only one of the three columns of the table is so ap- 

 parent as to need little, if any, explanation, but there are certain facts 

 relating to members of others of these families which such a table can 

 not distinctly show . Mention of such of these and kindred facts as is 

 deemed necessary for the present purpose is made in the following 

 supplementary notes. 



The greater part of the Buecinidae inhabit marine waters, but they 

 uotunfrequently range into estuaries, and certain species are sometimes 

 found in waters that are nearly or quite fresh. 



While a majority of the species of the Assiminiidse find a freshwater 

 habitat congenial, sonic live upon the borders of saline water, at river 

 mouths or in estuaries. 



Of the living forms referable to 'the Kissouhe some inhabit marine, 

 some brackish, ami others fresh waters, but no species is known to 

 range from one of these kinds of habitat into another, although it is 

 probable that some may <lo so. Because the fossil species referable to 

 this family may, as a rule, be regarded as distinct from all living spe- 

 cies, the presence of fossil shells referable to this family in any forma 

 tion does not necessarily give any definite indication as to whether it 

 was of marine, brackish, or fresh water origin. In such cases the char- 

 acter of the habitat must be determined by means of their faunal asso- 

 ciates. 



The Littorinuke usually inhabit the margins of marine waters, but 

 some species have a brackish-water habitat. 



The Melanopsida- usually inhabit fresh waters, but some of them are 

 ound in waters that are more or less saline. 



