BRYOZOA. 185 
Arth rostyllida. | 
zocecial 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 pore-like 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 lamin 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. af 
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 flabellata, but both 
the description and figures are entirely inadequate for anything like satisfactory 
identification. It may belong to Clathopora, but Stictoporella 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 locality.—A bundant in the middle third of the Trenton shales at Minneapolis and 
St. Paul, Minnesota. The species seems to be restricted to this horizon. 
Family ARTHROSTYLLIDA, 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. 
