BRYOZOA. 293 
Batostoma montuosum,] 
the zocecial walls seemingly in contact in most cases, the mesopores few, and the 
acanthopores indistinct. This condition, which is to be counted as rare, corresponds _ 
to fig. 18. From it the usual deviation consists in the rounding and constriction of 
the zocecial apertures, and widening of the interspaces. When carried to the extreme 
we have subcircular apertures enclosed by a thin peristome, separated by depressed 
interspaces in which the mouths of a few mesopores may be noticed. In all these 
conditions (corresponding to figs. 16, 19, 20 and 21) the average number of zoccial 
apertures in 3 mm. is nine or ten. The acanthopores also are rarely very conspicu- 
ous for size, though they are numerous enough and may be distinctly visible on well 
preserved examples. But in a variety, or rather an occasionally observed condition 
of the species, the acanthopores are much larger and situated three or four in the 
now thin walls of each zocecium, causing their apertures to be more or less 
indented at the points occupied by them. 
Internal characters: The extreme variability of tangential sections is well shown 
in the seven figures on plate XXV. And yet the species is an easy one to identify by 
means of thin sections. Each tangential section will show from two to four of the 
conditions illustrated, and although I used five specimens and six sections I might 
have obtained an equal range of variation with two of them. The greater part of 
them vary between the appearances represented in figs. 16, 19 and 21. Vertical 
sections are more constant, having numerous diaphragms—about four in the zocecial 
tubes and six in the mesopores in 0.5 mm.—in the peripheral region, and none in 
the axial. The usual irregularity of the tubes in the axial region is noticeable. 
Also a peculiar moniliform structure of the walls in the peripheral region is the 
probable cause of some of the variations noticed in tangential sections. 
The zoarial growth is less robust and the average size of the zoccia less than 
in B. fertile. The opposite is the case when compared with B. winchelli, which, like 
B. minnesotensis, has also much fewer mesopores. The species is evidently a fore- 
runner of B. humile of the Galena shales and B. jamesi Nich. sp. of the Cincinnati 
rocks. 
Formation and localityx—Moderately abundant in the middle third of the Trenton shales at Min- 
neapolis and St. Paul, Minnesota. 
Mus. Reg. No. 8135. 
BaTOSTOMA MONTUOSUM, 7. Sp. 
PLATE XXV, FIGS. 26-28, 
Zoarium ramose, small, branches dividing rapidly, 5 to 10 mm. in diameter; 
surface with more or less conspicuous monticules, 2.0 or 2.6 mm. apart, occupied by 
apertures of the same size as those in the depressions; the summit of the monticules 
