PALEOZOIC BRYOZOA 255 



ber until they are practically absent throughout the corresponding part 

 of the formation in Virginia, West Virginia, and Pennsylvania. What- 

 ever reason may be assigned for this northward diminution, the fact re- 

 mains that they are abundant in the south. Hence the inference is plain 

 that they must have been derived from that direction — in other words, 

 from the Gulf of Mexico and perhaps other contributory seas lying beyond 

 it. Other evidence tending to the same conclusion is that very similar 

 species of Bryozoa, about whose derivation from the preceding Pierce 

 fauna there can be no question, occur in the Lebanon, an upper Stones 

 Kiver formation, and again in the Lowville, the first of the deposits of 

 the Black Eiver group. The Lebanon Bryozoa, like those of the Pierce, 

 diminish rapidly northward in the Appalachian Valley; the Lowville 

 much more slowly, so that a very fair representation of the Tennessee 

 species of this formation is recognizable as far north as New York and 

 Canada. From this point westward to Minnesota they rapidly diminish 

 in number. Following the Lowville southward from Minnesota we find 

 that the Bryozoa, like the corals, are almost entirely absent. 



The three invasions of Bryozoa so far mentioned are undoubtedly from 

 the south. Doubtless the distribution of these organisms in the conti- 

 nental basins was favored by warm currents entering tlie inland seas from 

 the oceanic basins contributing the water. As previously stated, the 

 migration of the Bryozoa is largely confined to this mode of dispersal. 

 Considering the limited geographic distribution of the Pierce and Leb- 

 anon Bryozoa, the invading currents must have been of relatively small 

 importance in those seas. Physical data bearing out this view are at 

 hand. The wider distribution of the Lowville types suggests perhaps 

 more active and certainly longer sustained currents. 



That these currents passed up chiefly along the east shore of their 

 respective seas is likewise suggested by the distribution of the Bryozoa. 

 So far as known, Bryozoa are entirely absent in the Stones Eiver rocks 

 of northern Arkansas and Missouri, and either very rare or totally absent 

 along the western, northern, and eastern shores of the basin as far as 

 southern Virginia. As stated, the Bryozoa are also very abundant in the 

 Lowville, and that here again their number becomes less and less away 

 from the southeastern shore until in Missouri but a single species re- 

 mains, and this owes its presence to its parasitic habit on a shell whose 

 migration was less dependent on currents. 



Similar conditions of migration by currents is suggested by the Bryo- 

 zoa and corals of late Paleozoic ages. It is especially well marked by the 

 Onondaga Bryozoa, which are abundant all along the eastern shore up to 

 Ontario, but are almost entirely wanting on the flanks of the Ozark up- 



