A .V. . „.. BRYOZOA. 185 



Arthrostyllidje. | 



zooecial apertures are often closed by a convex plate, with or without a minute 

 subcentral perforation. 



Tangential sections show that the polygonal boundary of the zocecia is marked 

 by a dark line, which under favorable preservation will probably consist of a row of 

 exceedingly minute porerlike dots. Visceral cavity ovate or subcircular, generally 

 of less width than the walls. Mesopores numerous, of irregular shape and unequal 

 dimensions, often completely filled, or only preserving a very small central cavity. 



Vertical sections show that the divisional laminse are somewhat flexuous, the 

 tubes at first thin-walled and prostrate, overlapping each other for some distance, 

 that they subsequently bend abruptly outward, and that their walls at the same 

 time are much thickened and marked with oblique lines parallel with the form of 

 the apertures. The mesopores appear as narrow open spaces when not entirely 

 filled by the secondary deposit of sclerenchyma. Diaphragms and hemisepta 

 wanting. 



The Canadian Trenton limestone species identified by Billings with Eichwald's 

 Coscinum proavium, is closely related to S. cribrosa, but differs in having narrower 

 branches, subequal and more regularly disposed fenestrules, and less numerous 

 mesopores. Another cribrose species, but in every respect smaller than these, 

 occurs in the " Pierce " limestones at Murfreesboro, Tennessee. Still another Lower 

 Silurian bryozoan with inosculating branches has been described by Hall from the 

 Trenton rocks of northern Wisconsin.* He named it Clathropora flahellata, but both 

 the description and figures are entirely inadequate for anything like satisfactory 

 identification. It may belong to Clathopora, but Stidoporella is more likely to be 

 right. Then again it is not impossible that it is a Coscinella or even a Coscinum, 

 since in all of these paleozoic genera the zoarium is cribrose. Compared with the 

 characters shown in Hall's figures, it is evident that he had before him a more 

 robust species, with branches and fenestrules much larger than in S. cribrosa. 



Formation and iocaKt^.— Abundant in the middle third of the Trenton shales at Minneapolis and 

 St. Paul, Minnesota. The species seems to be restricted to this horizon. 



Family AKTHROSTYLLID^, Ulrich. 



This interesting family of small Bryozoa is strongly represented in the Lower 

 Silurian rocks of Minnesota. On account of the minute proportions of most of them 

 their dismembered zoaria are generally to be found only by searching the*surface of 

 the slabs of fossiliferous limestone that occur so abundantly in certain parts of the 

 Trenton shales. These are often full of the separated joints. The most satisfactory 



♦Foster and Whitney's Report, vol. 2, p. 207, 1851. 



