224 THE PALEONTOLOGY OF MINNESOTA. 
[Atactoporella insueta. 
zoarium. On plate XV, fig. 16 illustrates the characters as brought out in a section 
taken from a young example. The section drawn on plate XVIII, fig. 2, was pre- 
pared from the thickest specimen seen. Between these two figures the student will 
be able to work out the essential features without further comment. It might be 
well, however, to direct attention in the last figure to the infilling and contraction 
of the mesopores in the upper half. Also to the fact that some of the zocecial tubes 
in vertical sections, and the same remark applies to all Bryozoa having cystiphragms, 
may appear to be, in part at least, crossed by complete diaphragms. This appearance 
however, is merely the result of the different directions in which the cystiphragms 
cross the various tubes shown in the section. ‘To obtain the characteristically curved 
line of the cystiphragm it is necessary that the section pass nearly across them. 
When the section passes through the tube parallel with the inner edges of the cysti- 
phragms they must necessarily appear as straight or oblique lines, thus simulating 
true diaphragms. 
This variety differs from the typical form of the species in the following particu- 
lars: The zocecia are a little larger, the mesopores less numerous and often of larger 
size, the acanthopores one or two more to each zocecium, and the tabulation a little 
more compact. Compared with other species, A. typicalis is readily distinguished by 
the exceeding tenuity of the zocecial walls, and the greater projection inward of the 
acanthopores. The latter are more numerous than in any of the other Minnesota 
species known. 
Formation and locality.—The types of A. typicalis are from the Utica horizon of the Cincinnati sec- 
tion at Cincinnati, Ohio, and vicinity. The present variety precipta is from the middle third of the 
Trenton shales at Minneapolis, St. Paul, Fountain, and other localities in Minnesota. 
Mus. Reg. Nos. 5983, 5984. 
ATACTOPORELLA INSUETA, 7. Sp. 
PLATE XV, FIGS. 13-15; PLATE XVIII, FIGS 5-8. 
Zoarium forming thin crusts over shells, crinoid columns, and ramose Bryozoa, 
generally about 1 mm. in thickness. Surface with clusters of large cells at intervals 
of 3.8 mm., usually little or not at all elevated, at other times rising into low and 
broad monticules. Zocecial apertures in old stages subangular and with thin inter- 
spaces in which the mesopores are not readily distinguishable; in younger stages more 
rounded, often ovate, with the interspaces usually somewhat wider and the meso- 
pores obvious ; walls thin, but little inflected. In the commonest form of the species 
there are from thirteen to fifteen zocecial apertures in 3 mm., but in the variety 
illustrated in figs. 7 and $ on plate XVII, only eleven are to be counted in the same 
space. Acanthopores of medium size, two to four to each zocecium, situated in the 
