American Paleozoic Bryozoa. 255 
the two extreme phases above noted. Just after the cells assume an 
erect habit, they have a thin complete wall, and are separated from 
each other, often completely, by large interstitial cells. As the section 
gradually passes into more superficial regions, the cell walls seem to 
thicken, but, asa closer inspection will prove, it is done entirely at the 
expense of the interstitial cells, the secondary deposit being made 
upon the outer side of the walls of the true zoecia, until, just below the 
surface, the interstitial spaces having at the same time also become 
narrower, they (the interstitial cells) can only with difficulty yct be 
recognized. The true cells are somewhat irregular in outline, of un- 
equal size, and when cut at a level deep enough, always show the 
curved edges of the cystoid diaphragms. Except in the earlier stages 
of the zoarium, the spiniform tubuli are very numerous, and increase 
slightly in size with age. Figure 8a, on Plate XII, represents a por- 
tion of the section intermediate between the two extremes. 
Vertical sections (PI. XII.,-fig. 8) show that the tubes at first are 
prostrate, with thin walls, and a few straight diaphragms. As they 
assume an erect position, the interstitial tubes are developed, and if 
these are not too compactly filled by the secondary deposit, numerous, 
thongh not crowded, horizontal diaphragms will be observed in them. 
When, however, the filing is compact, the cell tubes have the appear- 
ance of being provided with walls of very unequal thicknesses. Short 
series of cystoid diaphragms, and a few straight ones, are present in 
nearly all of the true zowcia. The spiniform tubuli are easily recog- 
nized in well prepared sections of this kind. 
This species is in many respects closely allied to both A. mundula, 
Ulrich, and A. ortoni, Nicholson. From the first it is distinguished by 
its more robust zoarium, more compact monticules, more hirsute inter- 
tubular spaces, and less closely tabulated, but more compactly filled 
interstitial tubes. Nicholson’s species is distinguished by its more 
delicate zoarium, thicker interstitial spaces, more oblique, and regularly 
arranged cells. Each of the three species is persistent in attaching 
itself to some particular class of Jbjects. Thus, all of my specimens 
of A. multigranosa, are fixed to the shells of Orthoceras ; those of A. 
munduia, grow upon other bryozoa of ramose or frondescent form; 
while those of A. orton?, are all attached to some brachiopod, and in 
nine cases out of ten, the shells of Strophomena alternata, Conrad, 
have been selected. . 
Formation and locality: Cincinnati group. This is a rare species. 
and seems to be restricted to a few feet of strata, having an elevation 
