186 THE PALEONTOLOGY OF MINNESOTA. 
(Arthrostyllide. 
specimens, however, are those which are obtained by picking over the residue of 
washings of the shales themselves. These are better, not only because they can be 
studied from all sides, but because their preservation is, in most cases, more favorable. 
Unfortunately, I had neither the time nor the opportunity of making extensive 
washings of shales in Minnesota, and that method of collecting was employed to 
only avery limited extent. Here and there a pound or two of unusually rich clay was 
carried away and washed during leisure moments after my return home. One of 
these packets proved to cgntain so many interesting things, and withal was so rich 
in individuals, that it deserves mention. ‘The shale was from the lower part of the 
Galena shales, which, according to my reckoning, is the exact equivalent of the 
Trenton limestone of New York. After washing away less than half its bulk nearly 
two-thirds of the residue consisted of good fossils, of which the larger ones, mainly 
species of Prasopora, Homotrypa, Callopora, Constellaria, Eridotrypa and some Brachi- 
opoda, were separated by sifting the finer material away from them. A large pro- 
portion of this fine material consisted of small fossils, among them five or six species 
of Ostracoda, (most of them described in this volume as new) and at least eleven 
species of small Bryozoa. Of the latter eight belong to the family under considera- 
tion, two of them being species of Arthroclema, three of Helopora, and three of 
Nematopora. 
The jointed character, of the zoarium is the most conspicuous and perhaps also 
the most important feature of the family. It is well shown in all the genera except 
Nematopora, in which articulation occurs only at the basal extremity, the zoarlum 
above the base being a dichotomously dividing, continuous stem. In Helopora, Hall, 
Sceptropora and Arthrostylus, the segments are simple and terminally joined together, 
and doubtlessly formed bushy zoaria. But in Arthroclema, Billings, the zoarium forms 
a more or less plumose expansion, divisible into numerous primary, secondary and 
tertiary segments, those of the first and second order being connected terminally 
and ranged in straight lines. A deep socket occurs on one or two opposite sides of 
each of the strong joints of the primary series and a shallow one in most of those of 
the smaller secondary set, in which respectively the first of the series of the second 
and third order is inserted. 
The zocecia are arranged in a radial manner around a central axis and, excepting 
Arthrostylus in which one side is marked with longitudinal striae only, open on all 
sides of the subeylindrical segments. In transverse sections the primitive portion 
of the zocecia is wedge-shaped, but in longitudinal sections they often appear tubular. 
The length of the tubes depends very largely upon the diameter of the segment, since 
all of them reach the central axis. It is evident that the obliquity of the tubes also 
has something to do with their relative lengths. They are, however, not to be con- 
