138 THE PALEONTOLOGY OF MINNESOTA. 



[Eurydictya. 



zooecial apertures are to be counted in 5 mm ., while twelve rows occur on a branch 

 3.0 mm. wide, on which the non-poriferous borders occupy space amply sufficient to 

 accommodate another row on each side. 



Internal characters not observed, the process of fossilization having been too 

 unfavorable to preserve the minuter details of structure. 



The small footstalk, rapid spreading of the zoarium, and the wide marginal 

 space, are the characters relied upon in distinguishing this species. In other respects 

 the species is very near R. trentonensis and R. gmndis. 



Formation and locality. All the specimens seen were collected by the author from the lower lime- 

 stone of the Trenton formation, at Minneapolis, Minnesota. 



Mus. Reg. No. ? 5934 . 



Genus EURYDICTYA, Ulrich. 



Eurydictya, ULRICH, 1889. Miller's N. Amer. Geol. and Pal., p. 301 ; 1890, Geol. Surv. 111., vol. viii, 



pp. 389 and 520. 



Zoaria bifoliate, consisting of broad, simple or irregularly divided expansions, 

 the surfaces of which exhibit more or less conspicuous, though usually small, maculae 

 or monticules. Zooecia of the same type as in Rhinidictya. 



Type : E. montifera Ulrich, 1890. Geol, Surv. 111., vol. viii, p. 521. 



This genus was established for the reception of a small group of Lower Silurian 

 species that, though intimately related to Rhinidictya, Ulrich, it seemed desirable to 

 distinguish from that genus. The broad and undefined zoarial expansion pertaining 

 to the several species gives them a very different aspect from that presented by the 

 narow, parallel-margined, and regularly branching stipes so strictly adhered to by 

 all the true species of Rhinidictya. That intermediate forms occur is true, nor can 

 we doubt that the dividing line between the two genera will continually grow more 

 shadowy with the discovery of new species. But, as that difficulty is encountered 

 by the systematist throughout all organic nature, it cannot be regarded as a bar to 

 the formation of generic groups, because, theoretically, if the course were carried to 

 its logical conclusion, all necessity for classification would cease. Some recognition 

 of obvious departures from a type is necessary, and in the present incompleteness of 

 our knowledge the only satisfactory plan to accomplish this is to adhere strictly to 

 the binomial nomenclature. In this declaration I am to be understood as aiming at 

 subgeneric rather than varietal designations. 



Eurydictya multipora ( ? Hall's sp.), the only species of the genus so far known to 

 occur in Minnesota, is the least typical of the genus. In shape and structure of its 

 end walls the species approaches Phyllodictya varia. The type of the genus, E. mon- 

 tifera, maybe looked for in the upper beds of the Hudson River group in Fillmore and 

 other counties in the southern part of Minnesota where that horizon is exposed. 



