Cumings — Development of some Paleozoic Bryozoa. 75 



colonies arise in both the Fenestellidse and Polyporidge, and may 

 be produced with such persistence as to give rise to flabellate 

 genera. In other cases they occur sporadically, and undoubtedly 

 in its phylogenesis this peculiarity first occurred only in occa- 

 sional individuals. 



The carinas of Fenestella originate as septal upgrowths of the 

 basal plate, coinciding with planes of division between adjacent 

 zooecia. They are manifestly protective and strengthening 

 structures. 



82 



81 



Figure 81. — Stomatopora from the Trenton of Minnesota, showing the 

 propagation of the colony by simple linear budding, or by formation of two 

 lateral buds, x 6. 



Figure 82. — Proboscina from the Trenton of Minnesota, showing incipient 

 coalescence of branches, x 12. 



Figure 83. — Arthroclema from the Trenton of Minnesota, with transverse 

 and axial sections of another species. Compare with axial portion of fig. 55. 

 Sections x 13 ; other figures x 9. 



Unitrypa and Hemitrypa conform in their ana- and meta- 

 nepiastic stages to Fenestella, but differ from the latter in the 

 production, during- the paranepiastic stage, of a peculiar cup, 

 surrounding and embracing the basal stalk of the zoarium. 

 This cup represents the upgrowth of the margin of the basal 

 plate, and the fusion therewith of one or more series of primary 

 scalae, through acceleration of growth of the latter structures. 

 In their ontogenesis the scalse are intimately related to the 



