56 BIOLOGICAL SOCIETY OF WASHINGTON. 



known to have existed in those early periods, do not seem to indicate 

 so distinct a separation of the Paleozoic and Mesozoic as the plant 

 remains do; for the types of the former have come down to the 

 present day. The land snails, before referred to, furnish remarkable 

 examples of types of animal life which have passed from the Paleo- 

 zoic to the Mesozoic age, and thence down to the present time. 



Although the biological evidence SQems to be conclusive that, from 

 the earlier Paleozoic to the present time, considerable portions of 

 land have been continuously above the level of the sea within the 

 area which is now occupied by the North American continent, we 

 have no indication from such evidence as to the shape and dimen- 

 sions which those early land areas successively assumed. We are, 

 however, justified in the opinion that some of them were of great 

 extent. From a biological point of view, also, I regard it as prob- 

 able that extensive land areas formerly existed adjacent to that 

 which the present continent occupies, and that they have been lost 

 by submergence and erosion. This opinion, I think, derives strong 

 support from the apparent sudden beginning, and the equally sud- 

 den termination, of certain extensive faunas of highly organized 

 land animals, which are known to have existed and become extinct. 



I do not forget, however, that our knowledge in this respect is 

 slight, and that it is possible that the progenitors of these faunas 

 may have existed within the bounds of the present continent, and 

 that their remains may have been destroyed, so as to have escaped 

 the scrutinizing search that has been made for them. But in view 

 of all the known facts, both geological and biological, I at present 

 hold to the opinion just expressed. 



The existence of the immense and diversified dinosaurian faunas 

 of Mesozoic time, snbsisting, as they did, largely upon vegetation, 

 seems necessarily to imply the co-existence of large land areas : 

 also their apparent sudden introduction at the beginning of that age. 

 seems to make it necessary to conclude that their progenit-ors ex- 

 isted somewhere before the close of Paleozoic time, although no 

 remains of those progenitors have been discovered. Because no 



