186 THE PALEONTOLOGY OF MINNESOTA. 



[Arthrostyllidir. 



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 a very 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 contain 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 Heloporrt, 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 zoarium 

 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 zooecia are arranged in a radial manner around a central axis and, excepting 

 Arthrostylus in which one side is marked with longitudinal stria? only, open on all 

 sides of the subcylindrical 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- 



