BKYOZOA. 159 



Pachydictya.] 



The small, dwarfish appearance of the zoarium of this species will distinguish it 

 from all others of the genus known to me. In other respects the species resembles 

 P. acuta Hall, which occurs associated with it, but because of the much smaller size 

 of P. pumila confusion between them is rendered highly improbable. Another asso- 

 ciated species, Ehinidictya minima, is more likely to be confused with it, but after a 

 little comparative study, the student will find himself able to distinguish them 

 almost at a glance. The rather rigid and subcylindrical character of the stems of 

 the next described P. triserialis are sufficiently distinctive of that species, and render 

 further comparisons unnecessary. 



Formation and locality. Ease of the Galena shales, near Cannon Falls, Minnesota, where it is asso- 

 ciated with species of Nematopora, Arthroclema armatum, Helopora mucronata, and other small Bryozoa, 

 all of them characteristic of the horizon. A single example apparently referable to this species, was found 

 at the horizon of Phylloporina corticosa, and another occurs on a slab of Trenton limestone, from Trenton 

 Falls, New York. 



Mus. Reg. No. 8107. 



PAOHYDICTYA TKISBEIALIS Ulrich. 



PLATE X, FIGS, lt-14. 



Pachydictya triserialis ULRICH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 187, fig. 12. 



Zoarium consisting of very slender, parallel-margined, subcylindrical or com- 

 pressed branches, 0.6 to 0.8 mm. wide, and 0.3 to 0.55 mm. thick ; branches bifurcat- 

 ing at intervals of from 5 to 10 or more mm., oval or obtusely hexagonal in cross- 

 section, the margins never, or at any rate but rarely, acute, in most cases to be 

 described as narrowly rounded. Each face with three rows of longitudinally 

 arranged zooacial apertures, occasionally with a fourth row, but only for a short 

 distance. These rows are often not exactly parallel with the margins of the 

 branches, a faint tendency to arrangement in long spirals being perceptible in 

 those cases. Zorecial apertures elliptical, nearly twice as long as wide, largest in 

 young or worn examples, separated by intervals equal to their long diameter, with 

 from 11 to 13 in 5 mm.; occasionally enclosed by a delicate rim or peristome, but 

 oftener with sloping edges. Between the rows an obtuse ridge. Entire surface, 

 especially of the older portions, minutely papillose. Non-poriferous margins nar- 

 row, readily overlooked, generally wider on one side than on the other. 



Internal characters similar to those of P. acuta Hall, and P. pumila. In such a 

 small species the interstitial vesicles are necessarily reduced to a minimum, and 

 in this one the solid filling of the interspaces is so dense that their original presence 

 is not easy of demonstration. 



The subhexagonal narrow branches of this species present considerable resem- 

 blance to species of Nematopora like N. lineata (Helopora Billings). Of course, there 



