president's address. 57 



trace of the progenitors of either the dinosaurian faunas or dicoty- 

 iedonous floras of the Mesozoic age have been discovered in previ- 

 ously existing strata, I am disposed to conchide that those progeni- 

 tors originated upon, and occupied, land areas, which became 

 gradually submerged, together with their ancient faunal and floral 

 remains; while their living successors escaped by migration and 

 dispersion to adjacent and unsubmerged portions of land, which are 

 now within our continental area. 



There is reason to believe that during the Triassic and Jurassic 

 periods large continental areas were above the level of the sea, 

 within and near the present limits of the North American conti- 

 nent; but we know comparatively little of the terrestrial life of 

 those periods from actually discovered fossil remains. 



If we except the Paleozoic bivalve mollusca of supposed fresh water 

 origin, which have already been referred to, the remains of the 

 earliest fresh-water molluscan fauna, of which we have any satisfactory 

 knowledge, are found in Jurassic strata. These Jurassic mollusks 

 belong to well-known types now living in the fresh waters of this 

 continent ; and they are also so highly organized as to point back 

 to a still more ancient period, as that of their origin. These mol- 

 lusks suggest the existence during the Jurassic period of fresh-water 

 lakes and rivers within what is now Western North America, and 

 the lakes and rivers in turn suggest the existence then of a con- 

 siderable continental area. It is possible that those Jurassic rivers 

 were, in part, portions of river systems which had held a persistent 

 existence from former geological periods ; but we have no direct 

 paleontological evidence of it. This gill-bearing molluscan fauna 

 seems also to be ancestrally related to faunas which are known to 

 have existed in subsequent periods, as well as to certain fresh-water 

 mollusks now living in North America. 



A few species belonging to the fresh-water family Unionidae have 

 been discovered in Cretaceous strata of the western portion of this 

 continent, but they all appear to be of different types from any of 

 the family now living. I take this to be an indication that the 



