158 THE PALEONTOLOGY OF MINNESOTA. 



LPachydlctya. 



In the very young specimens, or at the distal extremities of the branches of the 

 more mature ones, the zooecial apertures are comparatively large, the lateral inter- 

 spaces correspondingly narrow, and the end spaces with one or two depressions. In 

 this stage the interspace granulations are very faint, but in the succeeding stages they 

 are much better defined, the apertures often smaller, with the width of the inter- 

 spaces increasing with greater rapidity, the increase in the circumference of the 

 branches being divided between the lateral interspaces. In most of these specimens 

 the interspaces are now flat or faintly concave, with a more or less distinctly recog- 

 nizable though thin peristome about the apertures. In others a row of the inter- 

 stitial papillae occupies a faint longitudinal ridge, that may be elevated to slightly 

 abave the level of the peristomes. In more rare instances the peristomes appear to 

 be wanting over parts of the surface, and the whole interspace convex and irregularly 

 granulose, and seeming to slope down into the apertures. These specimens have 

 quite a different aspect from the ordinary form of the species, indeed, so much so, that 

 I mistook them for a species of Bhinidictya. Non-poriferous margin never wide, 

 often so narrow as to be practically wanting. Its surface is papillose. Not infre- 

 quently large patches of the surface, where the zooecial apertures have been closed 

 by a thin deposit of calcareous material, are covered with such papillae. 



Internal characters vary much as in P. acuta, excepting that they are all a little 

 smaller, and the transverse walls between the prostrate cells of the zooecial tubes 

 straighter. 



When the preliminary description of this species was written 1 had unfortunately 

 mislaid the two specimens regarded as the types of the form named Ehinidictya 

 humilis, and which I believed to have been derived from the lowest shales at Minne- 

 apolis. In preparing the Minnesota material for my final studies they were found 

 and the label with them proves that they were really collected at the same time and 

 from the same beds as the original specimens of P. pumila. Later washings of the 

 shales from this locality have added greatly to the number of specimens. With 

 this more complete representation of the species I have become satisfied that the 

 supposed Ehinidictya exhibits merely another phase of surface marking of P. pumila, 

 deserving not even subordinate distinction. Among the lot, however, there is a form 

 of which I have over twenty specimens, that might be distinguished as var. sublata. 

 The zoarium does not appear to have been much larger than in the typical form, but 

 its branches are wider, and though there are generally two or three rows of zooacia 

 more than in the largest of the type form, the greater width of the branches is 

 chiefly due to a wide non-poriferous margin. 



