T. u ^, . . BEYOZOA. 145 



Pachydiotya.] 



This species is closely related to a common form of the Birdseye limestone in 

 central Kentucky, which I regard as likely to prove identical with Hall's Stidopora 

 lahyrinthica, described from the some horizon in New York. But in the absence of 

 any knowledge of the interior of that species, it would be highly injudicious, surely 

 unwarranted, to assert their identity. Still, it is possible that even the Minnesota 

 form may be only a local variety of that species. However, the probability of that 

 Supposition is so remote that I feel no hesitation in proposing the new name varla 

 for the form here described. 



Formation and tocai%.— Eestricted to the middle third of the Trenton shales at Minneapolis, Min- 

 nesota. A single example from about the same horizon at Cannon Palls. 



Mus. Req. No. 5953. 



Genus PACHTDICTTA, Ulrich. 



Pachydiotya Uleich, 1882, Jour. Cin. Soc. Nat. Hist., vol. v, p. 152; Foerste, 1887, Bull. Sci. Lab. 



Denison Univ., vol. ii, pt. ii, p. 162 ; Mulee, 1889, N. Amer. Geol. and 

 Pal., p, 313; Ulbich, 1890, Geol.Surv. 111., vol. viii, p. 390. 



This genus, in its fullest sense, falls into three distinguishable, yet not entirely 

 natural sections, having precisely the same relations to each other as Rhinidictya 

 and Eurydidya, Cystodidya, Dichotrypa and Prismopora. These genera, being based 

 entirely upon zoarial deviations, are evident to the unassisted eye, and the micro- 

 scope is not necessary in distinguishing them. To be consequent, a similar splitting 

 up of Pachydidya is suggested, but such a course would be only too liable to lead to 

 misunderstandings, since we would be obliged, for the same reason, to follow the 

 plan to its logical conclusion in dealing with Ptilodidya and Phcenopora, in which 

 precisely the same divisions, as well as others equally marked, obtain. It is, there- 

 fore, deemed sufficient for present needs to designate two of them with the non- 

 committal terms of Section a and Section h. The third, however, being a departure 

 in a more obvious and seemingly more important direction, is entitled to better 

 attention. For it the name Tkigonomotya. is proposed. 



The following diagnosis embraces the characters of the two sections, but those 

 features that may be considered as especially characteristic of one or the other, are 

 indicated by the letter a or Z> in parentheses following the statements. 



Zoaria bifoliate, consisting of irregular wide branches, large or small, and more 

 or less undulating, leaf-like expansions (a), or of narrow, subparallel-margined, and 

 dichofcomously branching stipes {b). Margins acute, with a non-poriferous border, 

 obliquely striate or grano-striate. Surface with small maculae and, about them or 

 taking their places, clusters of zocecia of more or less obviously larger size than the 

 average ; occasionally montiferous {a). In other cases ih) these clusters are repre- 



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