PALEOZOIC BRYOZOA 257 



Black Eiver or Decorali shales of Minnesota and Iowa. It has been rec- 

 ognized also at a number of localities in eastern British America and 

 extends southward in the Mississippi Valley to central Kentucky and 

 northern Tennessee. In the latter two States the fauna, which is in 

 ever3rwise typical, has been noted at only a few localities. Where ob- 

 served the bed is only from an inch to 2 feet in thickness and is limited 

 above and below by formations containing totally different faunas of 

 southern origin. Further, the upper and lower boundaries of this bed 

 exhibit clear evidence of interrupted deposition. Moreover, these ex- 

 treme southern wedges of the Decorah shale seem to be entirely confined 

 to shallow hollows in the underlying Lowville limestone. The facts are 

 essentially similar with respect to the second of these Arctic faunas, the 

 main difference being that whereas the first is entirely unknown, at least 

 in the northern Appalachian region, the second is locally developed in 

 central and eastern New York and extends thence southward into New 

 Jersey. Like the first, it is best developed in southern Minnesota and 

 northern Iowa, where it is found in the Prosser limestone, the proposed 

 name of a formation comprising the Clitambonites, Nematopora, and 

 Fusispira beds of the Minnesota reports. The third Arctic fauna is early 

 Silurian in age and belongs in the Eichmond series. It is marked es- 

 pecially by corals and Bryozoa, both of a decidedly Silurian aspect. It is 

 known in Baffinland and Alaska, and is locally found on tliis continent to 

 the south as far as southern Illinois, where it is included in the Noix 

 oolite or Edgewood beds. In the far West it is widely distributed, being 

 known in Arizona, Utah, New Mexico, and western Texas. 



That these forms really originated in the Arctic basin is indicated not 

 only by their distribution in North America, but also by the fact that 

 they are similarly developed in the Baltic province of Europe. As has 

 been clearly shown by studies of the Baltic Bryozoa just completed by 

 E. S. Bassler, a large proportion of species described from these beds in 

 Minnesota and elsewhere in America are represented by identical and 

 very closely allied species in Eussia. Up to date 70 out of 143 Minnesota 

 species of this class are found also in Eussia. 



To sum up, the Bryozoa have an exact bearing on paleogeography 

 (1st) because of the abundance of their fossil remains, (2d) because of 

 the certainty and comparative ease of ascertaining the critical characters, 

 (3d) their indication of the shallowness of the seas in which they lived, 

 (4th) their rapid and wide dispersal, justifying conclusions respecting 

 the essential contemporaneity of their occurrence, and (5th) the light 

 they throw on the direction and extent of marine currents. 



