BEYOZOA. 151 



Paohydiotya.l 



The foliaceous zoarium of this species will distinguish it from all associated 

 Bryozoa excepting Stictoporella frondifera. Both occur commonly on the same slabs, 

 and a careless collector might confound them. Still, after a little study, the differ- 

 ence in the size and shape of their respective zooecial apertures will become so evi- 

 dent that they may be distinguished at a glance. 



Formation and locality— Restricted to and very characteristic of the the lower third of the Trenton 

 shales. It is very abundant at Minneapolis and St. Paul, and has been found in greater or less abundance 

 marking this horizon in the shales at localities near Cannon Falls, Preston, Fountain and other points in 

 Minnesota. 



Mus. Reg. No. 5948. 



Pachydictya occidentalis Ulrich. 



PLATE VIII, FIGS. 20-27; PLATE IX, FIGS. 6-10. 



Pachydictya occidentalis Ulrich, 1886. Fourteenth Ann. Rep. G-eol. Nat. Hist. Sur. Minn., p. 75. 



Zoarium variajple, spmetimes consisting of narrow branches with subparallel 

 margins, in other cases spreading rapidly into slightly undulating fronds ; but the 

 commonest mode of growth is represented in figures 20 and 24 on plate VIII. In 

 these we have wide branches, with more or less divaricating margins, often of sub- 

 palmate form, with three or more small divisions above. Width 4 to 25 mm., 

 greatest thickness 1 to 2 mm. Margins acute, usually with a narrow, smooth or 

 finely striated border, best developed in the narrowest examples. The wider 

 specimens generally with only a single row of small solid spots having the same 

 structure as the non-poriferous border. These vary considerably in size, and are not 

 uniform even on the same specimen. In the narrow examples they are absent 

 except one or two just beneath each bifurcation. ^ Zooecial apertures elliptical, 

 arranged in comparatively irregular series, the longitudinal predominating. Curved, 

 transverse, and diagonally intersecting rows are also to be made out. Measuring 

 lengthwise, thirteen or fourteen in 5 mm.; transversely, seven or eight in 2 mm. 

 Interspaces generally rather narrow, but unequal. When an alternating arrange- 

 ment of the zooecial apertures prevails, the end spaces are decidedly the narrowest, 

 averaging in that case only about 0.1 mm., or scarcely more than half the width of 

 the lateral spaces. When however a transverse arrangement obtains t'hey are 

 nearlv equal at 0.13 mm. As a rule we may say that the shorter or transverse 

 diameter of the zooecial apertures is about equal to the width of the inl^rspaces. 

 Generally the interspaces are to be described as flattened, finely grano-striate, the 

 striae however, appearing to be irregular or interrupted at short intervals. In old 

 examples they may be convex, but in no case have I detected longitudinal ridges 

 between the rows of cells. Figure 26 represents one of a number of specimens, the 

 growth of which for some unknown reason has not been regular and continuous 



