THE BRYOZOA BASSLER. 



355 



is evident. Figure 6 gives a resume of the important types of both 

 fossil and recent Ctenostomata. The pinnately arranged stolons of 

 Rhopalon-aria usually represented by excavations in shells or corals 

 are perhaps the commonest of Paleozoic forms, although the chain- 

 like Allonema and the radially arranged bulbs of Ascodictyon are 

 occasionally found. The threadlike species are interesting in that the 

 oldest known bryozoan Heteronema priscum from the Lowest Ordo- 

 vician rocks of Esthonia is apparently of this type. In the Mesozoic 

 and Cenozoic eras few Ctenostomata have been found, and all of these 

 show great similarity to the Paleozoic Bhopalonwia. However, as 

 only their excavations are usually known, it is probable that the 

 structure of the zooecia was quite different. 



Fig. 6. — Fossil and recent Ctenostomata. 



1-2. Heteronema priscum Bassler. The oldest known bryozoan, a ctenostome from the 

 lowest Ordovician (Uugulite sandstone) of Esthonia ; (1) a colony attached to a 

 brachiopod shell, X3; (2) parts of two colonies, X22, with one growing over the 

 other. 



3-4. Vinclla repens Ulricb, Middle Ordovician shale of Minnesota ; (3) two colonies 

 attached to a brachiopod shell, § natural size ; (4) portion of a zoarium, X 12, show- 

 ing a nucleus with five divisions of the tubular stolon radiating from it. The pores 

 mark the points where the zooecia were attached. 



5. Vinella radialis Ulricb. Upper Ordovician, Cincinnati, Ohio. Four colonies attached 

 to a cephaloped shell, two-thirds natural size. 



