54 BIOLOGICAL SOCIETY OF WASHINGTON. 



continent begins much further back than the earliest period in the 

 strata of which we have yet discovered any indications of land ani- 

 mals. It is probable that land animals of the simpler kinds co-ex- 

 isted with the first established land vegetation ; but with the ex- 

 ception of some imperfect remains of a species of land snail, and 

 those of a few insects, which have been found in Devonian strata, 

 the remains of the earliest known land animals have been found in 

 strata of Carboniferous age. These Carboniferous land animals 

 comprise a few batrachians, insects, and air-breathing mollusks; 

 and although they are so few, they evidently represent portions of 

 a large and varied fauna which then existed. They are also so 

 highly organized and so diversified in character as to indicate that 

 they originated in genetic lines which began in earlier periods, 

 more or less remote. 



The Carboniferous air-breathing mollusks referred to are both 

 land and pond snails, and they have been found in widely separated 

 portions of North America. They all belong to types which are 

 represented by mollusks now living upon this continent, and by 

 those also which are known to have existed here at intervening: 

 periods. These facts seem to indicate plainly that land surfaces of 

 considerable extent have been continuous from that early period to 

 the present; but they tell us nothing yet of continental movements 

 which may have taken place in the meantime. 



Our knowledge of the relations of the different portions of living 

 non-marine moUuscan faunas would naturally lead us to suppose 

 that fresh-water gill-bearing mollusks existed simultaneously with 

 those ancient air-breathers. Nevertheless, with the exception of 

 certain bivalves, which have been found in Devonian strata, and 

 others in the Carboniferous, which have been doubtfully referred 

 to a fresh-water origin, fresh-water gill-bearing animals are not 

 known to have existed before the beginning of Mesozoic time. It 

 is, however, reasonable to suppose that such animals did exist in 

 Paleozoic lakes and rivers, although no satisfactory traces of them, 

 or of such bodies of fresh water, have ever been discovered. 



