New Lower Silurian Bryozoa. 



87 



of more mature ones, the longitudinal interspaces between the 

 zooecial apertures may show one or two pits or vesicles. 



Internal structure in conformity with that required by the genus. 



Pachydictya triserialis, the next described, is the only other very 

 small species of the genus known to me. These two can not be 

 confounded. P. acuta, Hall, is larger in every respect. None of 

 the other species are closely related. It is associated with Rhini- 

 dictya minima,. Ulr., but as soon as the differences have been 

 grasped they will be distinguished at once. 



Position and locality : Top of Trenton Shales, at Cannon Falls, 

 Minn., associated with numerous other small Bryozoa. 



PACHYDICTYA TRISERIALIS, n. sp. 



Fig. VI.— Pachydictya triserialis, n. sp. a, small portion of a piece of Trenton lime- 

 stone showing several of a group of branches of this species, all belonging to a single 

 zoarium. b, portion of one of the branches xl8. showing the arrangement of the 

 three rows of zooecia and the minutely granulo-striate character of the interspaces. 

 Most of the zoctcial apertures are closed by perforated or entire operculadike 

 structures, c, transverse section of a branch x25, showing the median tubuli along 

 the divisional central line. 



Zoarium, consisting of very slender, parallel-margined and but 

 little compressed branches, bifurcating at intervals of from 5 to 10 

 or more mm. Branches oval, or obtusely hexagonal in cross- 

 section, the margins never being acute, and at the most may 

 sometimes be described as narrowly rounded. Each face with 

 three rows of longitudinally-arranged zooecial apertures. The 

 latter are elliptical, about 0.2 mm. long and 0.1 mm. wide, sur- 

 rounded by a scarcely appreciable peristome, and often preserve 

 centrally perforated covers; the interspaces between the ends of 

 the apertures are longer than usual, and only eleven apertures 



