332 PALEONTOLOGY OF ILLINOIS. 



between the ends of the cells are directed obliquely upward and 

 outward from the mesial line, instead of at a right angle. P. ? 

 dawsoni, agrees also with Fenestella in having only two series 

 of cells separated by a mesial carina, and the branches united 

 by non-poriferous dissepiments. Taking all these characters 

 into consideration, I think we are justified in regarding P. ? 

 dawsoni as one of the ancestral types of Fenestella. On the 

 other hand, the rather prominent and but slightly contracted 

 cell-orifice point to a relationship with the Cyclostomatous 

 genus Protocrisina, sections of which are figured on plate 53. 



A careful study into the minute structure of the various spe- 

 cies of Phylloporina almost forces the conviction upon us that, 

 at some time before the commencement of the Trenton epoch, 

 there existed Bryozoa \which combined characters that during 

 subsequent ages became separately specialized and characteristic 

 of widely different groups. Take for instance P. trentonensis 

 Nicholson sp. Sections of this remarkable form remind us very 

 strongly of typical TREPOSTOMATA, the angular cells, their long 

 "immature" region, and the closely tabulated mesopores, all 

 being present in species of that suborder. The resemblance is 

 heightened in vertical sections of P. corticosa, by the addition 

 of diaphragms in the tubular zooecia. The last species has also 

 a very prominent mesial carina running along the centre of the 

 branches very much as in Semicoscinium. The relationship 

 thereby suggested is, however, in no wise born out by further 

 comparison. 



In P. aspera Hall, P. reticulata Hall, F. dawsoni n. sp. and 

 P.asperato-striata H.,the zooecial tubes are shorter, approaching 

 the Cryptostomatous cell. In the last species the interstitial 

 spaces are occupied by angular mesopores which have been 

 filled by secondary deposits of sclerenchyma. In Drymotrypa 

 dichotoma n. sp. we have a species in which the zooecial struc- 

 ture is not unlike Phylloporina, but the branches bifurcate fre- 

 quently and are not united by anastomosis as in that genus. The 

 general aspect, therefore, is not at all unlike that of a small 

 species of Thamniscus. That genus, however, has the short 

 cells characteristic of the FENESTELLID^E and never has dia- 

 phragms. There are some points also which suggest comparison 

 with Protocrisina. 



