12 NON-MARINE FOSSIL MOLLUSCA. 



unexpectedly large proportion of the known fossil pulnionate mollusca 

 are those whose habitat was constantly upon the land. The conditions, 

 however, which prevailed during the Ooalmeasure period of the Car- 

 boniferous age, and under which the immense quantities of vegetable 

 material that we now know as coal were preserved, were necessarily some- 

 what favorable to the preservation of such land mollusca as may have 

 found a habitat among that vegetation. The paucity of the remains of 

 such mollusca that have yet been discovered in the extensive coal-bear, 

 ing strata of that early period seems to prove that they could not then 

 have been very abundant ; but the discoveries of Dawson, Bradley, and 

 Whitfield show conclusively that a well-developed and widely differ- 

 entiated land moluscan fauna existed at least as early as the middle of 

 the Carboniferous age, and probably much earlier. 



Prom the Coalmeasure period until that of the Laramie the few re- 

 mains of non-marine mollusca that have been found in North American 

 strata present indications that the layers in which they were discovered 

 were deposited under estuary, palustral, or limited lacustrine conditions, 

 reference to which will be made in connection with the separate men- 

 tion of the species on following pages. The conditions which prevailed 

 in Western North America during the Laramie and Eocene periods 

 have already been indicated, and for fuller details the reader is referred 

 to the works before cited. 



Although there are really many facts now known which throw light 

 upon ihe physical conditions that prevailed, and the molhiscan faunae 

 which lived in Western North America during the various geological 

 periods from the later Paleozoic to the present time, a part of which 

 have been referred to, the following counter-facts should also be men- 

 tioned, because they show how far from perfect or continuous the geo- 

 logical record really is, in relation especially to the non-marine mollusca. 



Eivers, ponds, and marshes have necessarily existed ever since any 

 considerable portion of the continent rose above the sea, and those 

 rivers and ponds, without doubt, all had their own molluscan fauna? ever 

 since the later portion of Paleozoic time, if not from a still earlier date, 

 and yet no trace of any river deposits, except those of estuaries (and 

 few of these are known), has yet been discovered which pertain to any 

 geological epoch except that of the Post-Tertiary. The same can hardly 

 be said of palustral deposits, because much, if not all, of the coal must 

 have been produced under palustral conditions; and yet it is a note 

 worthy fact that the greater part of the known fossil palustral mollusca 

 have been found preserved in lacustrine deposits together with mol- 

 lusks of lacustrine origin, and very few in true palustral deposits 



Although it is only in the eastern half of the continent that any re- 

 mains of non-marine mollusca have been found in strata of Paleozoic 

 age, the remains of such mollusca as have been found there in strata of 

 any of the periods betweeu that of the Coalmeasures and the Post-Ter- 

 tiary are few aud unimportant.* 



* See remarks on a following page on spurious and doubtful species. 



