American Paleozoic Bryozoa. 245 
AMERICAN PALAOZOIC BRYOZOA.’ 
| Continued from Vol. vi., p. 168.| 
ATACTOPORA, Ulrich, 1379: 
(Jour. Cin. Soc. Nat. Hist., Vol. ii., p. 119.) 
More recent and thorough investigations, and our more advanced 
and constantly increasing knowledge of these intricate Paleozoic 
Bryozoa, have shown me that this genus, as originally defined by me, 
included at least two, unequal, and in many respects widely different 
eroups of species, besides a single species (A. septosa), which I have 
already referred to the genus Amplexopora. ‘The type species, A. hir- 
suta, unfortunately, is a member of the smaller group, the only other 
described species certainly known to have the characters of the genus, 
as I now propose to restrict it, being the A. maculata, a description of 
which was published by me in the same paper above referred to. That 
my subsequent remarks may be the better understood, I will imme- 
diately subjoin the emended definition of the genus, to conform with 
the restriction proposed. 
Atactopora (Restricted). 
Zoaria parasitically attached to foreign bodies, over which they form 
thin and expanded crusts, the thickness of which varies according to 
the number (rarely more than three) of superimposed layers, the latter 
having an approximately definite thickness. Zocecia with more or less 
thick walls, and indented or floriform orifices, due to the position of 
the numerous spiniform tubuli, which in the thick-walled species are 
developed almost exclusively in the poztion of the walls immediately 
_ surrounding the visceral cavity. True interstitial cells are wanting. 
At regular intervals the surface presents subsolid elevated * macule,” 
that are granulose and finely pitted, and apparently composed of 
peculiarly modified or aborted cells. Diaphragms few, always hori- 
zontal, 
As before stated, the type of the genus is A. hirsuta, the internal 
_ structure of which is represented on Plate XIL., by figs. 1 and la. The 
first represents a portion of a tangential section, enlarged eighteen 
diameters, and shows very distinctly the characters of the species as 
they are brought out in a section of this kind. The cell-inter- 
spaces are thick, and the walls of adjacent tubes are apparently fused 
