270 bulletin: museum of comparative zoology. 



WiERLAND GROUP. 



This series, held together by the presence throughout of Echino- 

 sphaerites aurantium, shows a decided change of faunas from bottom to 

 top. The fauna of the oldest formation, the Dubowiki, is dominated 

 by its asaphids, but tl^ese trilobites immediately lose their importance, 

 and though they continue through the remainder of the Ordovician, 

 they are present in limited variety and numbers. As previously 

 stated, there are thirteen species which pass over from the AValchow 

 and Kunda into the Dubpwiki, but very few of them survive beyond 

 this formation. 



The Wierland fauna is, however, in general, an outgrowth of that 

 which preceded it in the same area. Among the trilobites, notable 

 new arrivals in this group are: — Chasmops, Sphaerocoryphe, Pseudo- 

 sphaerexochus, Sphaerexochus, Acidaspis, Hoplolichas, Homolichas, 

 Cyphaspis, Lonchodomas, and Ogygites (Basilicus of Schmidt). All 

 of these genera, with the exception of Acidaspis and Cyphaspis could 

 have arisen as modifications of types already in this region, so that we 

 have, as true invaders only these two genera. 



Among the brachiopods the important new genera are Christiania 

 andOxoplecia. The place of origin of these genera is unknown, but 

 from their short ranges and limited variety in Russia, it seems probable 

 that they are present there as migrants. Other brachipods intro- 

 duced here are Plectambonites and Rafinesquina, but the dominant 

 forms are the Clitambonites and Porambonites which continue from 

 the formations below. 



Among the gastropods, Bucania, Cymbularia, Eccyliopterus, 

 Salpingostoma, and Subulites make their appearance first in this 

 formation; while of the echinoderms, we find here the oldest species 

 of Caryocystites, Echinosphaerites, Cryptocrinites, Cystoblastus, 

 Cyathocystis, Hemicosmites, Hybocrinus, and Protocrinites. Among 

 the sponges, Receptaculites is introduced at this time. 



In running over this list of genera newly introduced into the Russian 

 Ordovician during AVierland time, we are struck by the fact that we 

 are- here dealing with more familiar genera. Barring some of the 

 cystids and one or two other genera, all these genera are known in 

 America, and most of them from the Middle Ordovician. The greater 

 part of these genera seem to have developed in the North European 

 basin and to have migrated thence to America. 



Certain of the genera are, in America, restricted to a belt along the 



