288 THE PALEONTOLOGY OF MINNESOTA. 
[Batostoma, 
Genus BATOSTOMA, Ulrich. 
Batostoma ULRICH, 1882, Jour. Cin. Soc. Nat. Hist., vol. v, p. 154; 1890, Geol. Sur. Lll., vol. viii, pp. 
379, 459; Foorp, 1833, Contri. Micro-Pal. Cam.-Sil. rocks Can., p. 17. 
Zoaria irregularly ramose, with a large basal expansion. Zocecial walls thin, 
and irregularly flexuous in the axial region, more or less thickened in the peripheral. 
In the most typical species the walls are irregularly ovate, thick and ring-like in 
tangential sections, with neighboring zocecia in contact only at limited points, the 
mesopores numerous, Closed at the surface, and irregular in shape and size, and the 
acanthopores abundant and with a larger central cavity than usual. Species vary 
from these to forms having polygonal, thin-walled zocecia and very few mesopores 
and acanthopores. Diaphragms strong, horizontal, complete, few or wanting in the 
axial, more or less abundant in the peripheral region. In the axial region of trans- 
verse sections the tubes are divisible into two sets, one larger than the other. 
Type: B. implicatum Nicholson, sp. 
Beginning in the Birdseye limestone and shales of Minnesota with B. fertile, 
B. magnopora, and an undescribed species, it is evident that, the same as with 
Callopora and Dekayella, the primitive stock of the genus was of the simplest type. 
Mesopores were few, the zocecia angular and thin-walled, and acanthopores both 
few and very small. And yet the first of these species varies by most gentle degrees 
to forms (B. fertile var. circulare) having numerous mesopores, circular zocecial aper- 
tures, and thick walls—in short to a form that approximates the most typical and 
fully developed species of the genus. 
In B. implicata and B. jamesi Nich. sp., of the lower half of the Cincinnati group, 
the zocecia are characterized by thick ring-like walls, irregularly oval apertures, 
numerous mesopores and acanthopores. Although closely allied, B. variabile differs 
conspicuously from them in having few mesopores and polygonal zoccia. Being a 
later species, and one that doubtlessly is a direct descendant of those forms, we are- 
justified in assuming that a reversion in structure toward the primitive type took 
place during the closing times of the Lower Silurian. While such a tendency seems 
to have been general it was by no means universal, since at certain localities (Stony 
Mountain, Manitoba, Iron Ridge and Delafield, Wisconsin) where the conditions for 
the development of these Bryozoa seems still to have been eminently favorable, the 
genus is represented by species with abundant mesopores. 
A similar reversion had already taken place during the deposition of the Utica, 
in which two of the species, both as yet undescribed, are so simple in their structure 
that their really intimate relations to B. jamesi might not be suspected. It is only 
