BRYOZOA. 995 
Atactoporella crassa.] 
angles of junction and between the angles in the walls of both the zocecia and meso- 
pores. In most cases the acanthopores are not very conspicuous at the surface. 
This is in part due to attrition, since on protected spots they are distinct enough, 
giving the surface a minutely granulose character of somewhat coarser and looser 
pattern than in A. typicalis. In a variety represented by a dozen or more specimens 
these structures are much more strongly developed. Indeed, these specimens 
remind one very much of A. schucherti Ulrich, so far known only from the upper beds 
of the Cincinnati group in Ohio. 
Internal characters: These are so well illustrated on the two plates above cited 
that detailed descriptions are rendered unnecessary. It will suffice to say that the 
mesopores, though usually rather large, are variable in size and distribution, that 
the zocecia in the spaces between the aggregations of large cells are often in contact 
with each other for more than half their circumference, that the walls are a little 
thicker than in A. typicalis, the acanthopores less numerous, larger, and yet not pro- 
jecting inward so much, while the tabulation of both sets of tubes is much less 
compact. 
As has been intimated, two varieties might be distinguised from the typical 
and more common form of the species. The first differs in having larger zocecia, the 
second in having. much stronger acanthopores, these being larger and prominent 
enough at the surface to obscure the view of the zocecial apertures. The latter may 
be compared with A. crassa, the following species, but they are too distinct in verti- 
cal sections to be confused. 
Formation and locality—Middle third of the Trenton shales at Minneapolis, St. Paul, Fountain, and 
other localities in Minnesota where that horizon is exposed. 
Mus. Reg. No. 5985, 
ATACTOPORELLA CRASSA, 2. Sp. 
PLATE XV, FIGS. 18-21. 
Zoarium a small, irregularly hemispheric mass, growing upon some foreign object, 
2 or 3 cm. in diameter and 5 to 10 mm. in hight. Surface without monticules, nor 
have distinguishable clusters of either large or small cells been detected. Zocecial 
apertures subangular, rounded or irregularly outlined, inclosed by thick walls, prom- 
inently elevated at most of the angles of junction into strong acanthopore spines ; 
about fourteen apertures in 3mm. Mesopores less numerous than usual, small and 
difficult to detect at the surface. 
Internal characters: Considerable variety of structure, depending upon age, is 
exhibited in different tangential sections or in parts of the same section. These, as 
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