BRYOZOA. 223 
Atactoporella typicalis.] 
ATACTOPORELLA TYPICALIS, Var PRACIPTA, nN. var. 
PLATE XV, FIGS. 16 and 17; PLATE XVIII. FIGS, 1-4. 
This form, though much earlier, is too much like the Cincinnati A. typicalis 
Ulrich (Jour. Cin. Soc. Nat. Hist., vol. vi, p. 248, 1883), to be distinguished specifically. 
The resemblance is so close that the original description may, with a few trifling 
alterations, be made to fit the Minnesota form. Hundreds of feet of strata, however, 
intervene between the two horizons respectively held by the two varieties, in which 
the species is unknown. It would seem, therefore, to be a case of reapparition, not 
necessarily of the same species, but of the same type of structure, similar to the cases 
of Callopora and Dekayella noticed in this work. 
The characters of var prwcipta are as follows: 
Zoarium forming small thin crusts, rarely exceeding | mm. in thickness, over 
ramose Bryozoa and shells. Surface minutely spinulose, without monticules, except 
in rare instances (see plate XVIII, fig. 4), but exhibiting at intervals of about 2.5 mm., 
measuring from center to center, clusters of cells slightly larger than the average 
between which the interspaces are also a little thicker than usual. Zocecial apertures 
floriform, the walls thin and at each inflection raised into a small spine, the surface 
extension of an acanthopore, arranged in moderately regular, diagonally intersecting 
series, averaging fourteen in 3mm. Interspaces narrowing with age, very thin, with 
the zocecial walls largely in contact, the apertures direct and the mesopores small 
and easily overlooked in fully matured examples; thicker, with the mesopores more 
distinct and the zocecial apertures drawn out obliquely in younger stages of growth. 
Acanthopores numerous, small but sharply elevated, situated in the zocecial walls, 
four to seven, usually five or six, around each aperture. 
Internal characters: In tangential sections the zocecial walls are very thin and 
indented more or less sharply at from four to seven points in their circumference 
These inflections of the wall are emphasized by the acanthopores, one of which 
occurs at each point and appearing in nearly all cases to be formed onthe inner 
side of the wall. A few of the zocecia may be completely isolated by the interven- 
tion of irregularly-shaped mesopores, but as a rule they are in contact at limited 
points. The mesopores never form more than a single row, and their walls are 
entirely without acanthopores. The crescentic cut edges of the eystiphragms, some- 
times two or even three in each, are to be seen in each zocecium. On account of the 
nearly equal thickness of these edges and of the walls of the zocecia and mesopores, it 
is often difficult to discriminate between the various lines Shown in tangential sec- 
tions, Good vertical sections are difficult to prepare, because of the tenuity of the 
