American Paleozoic Bryozoa. 20 
interstitial spaces are slightly wider than ordinary., Cell apertures 
rounded, with more or less inflected margins, and somewhat unequal 
interstitial spaces ; twelve to fourteen may be counted in the space of 
.l inch; they are arranged in intersecting series, but these are not so 
regular as in most of the other species. Interstitial cells numerous, 
though, with the exception of a few instances in the monticules, they 
never completely isolate the true cells. Spiniform tubuli small, numer- 
ous, and apparently only developed on the margins of the true cells. 
Tangential sections (Pl. XII, fig. 6) show that the true cells have 
thin walls, that they are always in contact at one or more of their 
sides, and occasionally on all sides, that from three to six spiniform 
tubuli, of moderate size, surround each aperture, and that these 
structures are not developed in the interstitial spaces, but only within 
the walls of the proper zocecia, which, furthermore, are always more 
or less inflected at each point of their occurrence. The interstitial 
cells can not always be recognized, as they are often filled, especially 
just below the surface of fully matured specimens, by a secondary 
deposit of sclerenchyma. When not so filled, they can be, plainly 
enough, seen to occupy all the interstices between the thin walls of the 
true cells. Lastly, the crescentic edges of cystoid diaphragms may be 
detected in all sections of this kind. | 
Longitudinal or vertical sections (Pl. XII, fig. 62) show that the 
true tubes in their initial stages are prostrate, and that, as soon as they 
begin to assume an erect position, the interstitial tubes, and spiniform 
tubuli make their appearance. The walls are thin, and near the sur- 
face have a peculiar granular appearance (the same as is tound ina 
number of other bryozoa), for which I am not yet able to offer an 
explanation, unless it should indicate a porous condition of the walls. 
Numerous horizontal diaphragms cross the interstitial tubes. Beside 
a short series of cystoid diaphragms, which may be detected in most of 
the proper zocecia, a few straight diaphragms are also present. When 
the colony consists of more than one layer, the characters above 
described are repeated in each. 
By comparing this description, and those following, of 4. multi- 
granosa, Ulrich, and A. orton, Nicholson, with my original ones 
(loc. cit.), several differences will be noted. These may or may not be 
excusable, as they are mainly due to the fact that I was at that time 
not ingenious enough to prepare satisfactory sections of these thinly ex- 
‘panded bryozoa. Had I done so, I certainly would not have stated that 
interstitial tubes were entirely absent in A. multigranosa, and A. orton, 
