EARLY PALEOZOIC BKYOZOA OF THE BALTIC PEOVINCES. 177 



clusters of mesopores, or of cells larger than the average, at regular 

 intervals among the zocecia is another characteristic of the order, 

 although this feature is not confined exclusively to the Trepostomata. 



Although under the original definition of the Trepostomata the 

 change in the character of the zooecial tubes as they pass from the 

 simple immature stage to the more highly organized mature zone was 

 the most distinctive character, it must be remembered that the 

 Paleozoic Ceramoporidse and Fistuliporidse and the more recent 

 Cerioporidse have this same feature. In the latter families other 

 features, particularly their minutely porous wall structure, ally them 

 more closely to the Cyclostomata. 



Ulrich and Bassler, in their "Revision of the Paleozoic Bryozoa," ^ 

 have proposed two divisions of the Trepostomata based upon the 

 minute structure of the walls separating adjoining zooids. Of the 

 seven families now recognized under the Trepostomata, four have 

 the calcareous investment of adjoining zooecia amalgamated together 

 so that one wall can not be distinguished from its neighbor. In the 

 remaining three families the walls retain their duplex character, and 

 when the zocecia are adjacent their boundaries are marked by a dark, 

 divisional line. This line in all probability represents the fossilized 

 remains of animal matter which filled this space during the life of the 

 organism. Occasionally this narrow, intervening area is occupied 

 by a light-colored tissue, and in this case the outer boundaries of the 

 walls of each zooecium can be seen. In certain genera of both divi- 

 sions the amalgamation or the distinct character of the walls is difficult 

 to determine, especially when mesopores are numerous, but if the 

 zooecia are in actual contact there is little trouble in deciding the 

 position of the particular form under study. 



Following is a classification of the Ordovician and early Silurian 

 genera, with the genera common to the American and Baltic deposits 

 marked by an asterisk, and those restricted to the latter by a double 

 asterisk: 



Division AMALGAMATA XTlrlcIi and Bassler. 



Trepostomata in which the boundaries of adjacent zooecia are 

 obscured by the more or less complete amalgamation of their walls : 



Family MONTICULIPORID^ Nicholson . (emended Ukich). 



* Monticulipora D'Orbigny. ^Homotrypella Ulrich. 



*Orhignyella Ulrich and Bassler. Prasopora Nicholson and Ethe- 



Atactoporella Ulrich. ridge. 



Peronopora Nicholson. * Mesotrypa Ulrich. 



^Homotrypa Ulrich. Aspidopora Ulrich. 



I Smithsonian Miscellaneous Collections, vol. 47, 1904, pp. 15-55. 



