F. R. Cumings — Development of Fenestella. 175 



represented only by its degenerative stage, that is, by its latest 

 growth stage, all the earlier growth stages having been crowded 

 out or back into the larval stage. 



In accordance with this interpretation of bryozoan develop- 

 ment, the large size of the protcecium in ancient types is ex- 

 plicable and is thought to be due to a less degree of acceleration, 

 the calcification of the zocecial wall of the primitive individual 

 being allowed to proceed nearly to completion before the second 

 zooecium was superposed upon it. The probability that the first 

 polypide remains in the protcecium in Fenestella, instead of 

 ascending into the ancestrula as in modern Cyclostomata, may 

 indicate a still more primitive condition. The relations of the 

 protoecium and ancestrula in the Cyclostomata and in Fenestella 

 suggest the normal relation of superposition of the zocecia in the 

 Trepostomata. It is not without interest to find evidence, in the 

 development of Paleozoic Bryozoa, of the fundamental relation- 

 ship of these great groups. Ulrich (Geol. Surv. Illinois, vol. 

 viii) has already suggested such a relationship on the ground of 

 the resemblances of such types as the early Fenestellas, Phyllo- 

 jporina and Protocrisina. The evidence presented by these 

 adult types is greatly strengthened by the striking parallelism 

 of the nepiastic stages of Fenestella with the series of adult 

 types named above. 



Paleontological Laboratory, Indiana University, 

 June, 1905. 



EXPLANATION OF PLATES. 

 Description of Figures* 



Letters having the same meaning for all the figures : — 

 a, b, c, d, e, primary carina? (except figs. 17, 24, 47, and 48). 

 /, fenestrule. 

 k, carina. 



0, protoecium. 



s, substratum of bryozoan colony. 



z, z , etc., zocecia of generations later than the primary zocecia. 

 A, ancestrula. 



1, primary bud. 



II, bud of second generation, that is, derived from a primary bud. 

 Ill, bud of third generation. 



2, left lateral bud. 



3, right lateral bud. 



23, right lateral bud of the second generation, derived from a left lateral 



primary bud. 

 32, left lateral bud of the second generation, derived from a right lateral 



primary bud. 



* All drawings except figures 1-16 were made with the camera lucida. 

 Figures 30-32 are after Barrois, and figure 53 is after Ulrich. All the speci- 

 mens of Fenestella are from Thedford, Ontario. 



