BEYOZOA. 27S 



Var. multipora.] 



the rank of a species. Variety multipora, however, though often very distinct (com- 

 pare figs. 43 and 45, plate XXIII), is too intimately connected with the typical form 

 of the species to admit of greater than varietal distinction. 



My reasons for employing the adopted classification instead of the one just 

 suggested, "and which I really believe to be the best, are dominated by the fact that 

 very similar variations are encountered in large collections of D. ulrichi Nicholson, 

 sp., an exceedingly common species at Cincinnati, Ohio. This fact makes it, I think, 

 not only desirable but necessary that these Minnesota lower Trenton forms be 

 studied in coijnection with the various Cincinnati types qf the genus. The most 

 important point to be determined by such a study relates to the origin of the Cin- 

 cinnati varieties mentioned as being similar to those here separated from prcenuntia. 

 Did the two sets of varieties have a separate origin, or did those above defined con- 

 tinue and develope into the supposed varieties of D. ulrichi ? Although these ques- 

 tions, whose final solution would require months of careful labor, cannot now be 

 answered definitely, sufficient data have accumulated incidentally to render it more 

 than probable that a separate origin for the two sets is more nearly the truth of the 

 matter. In other words, 1 believe that future investigation will prove that D. ulrichi 

 was developed from some descendant of D. prcenuntia, that the known varities of the 

 latter became extinct before the close of the Trenton, and that in the Utica and 

 Hudson river eras a new set of forms was developed from the D. ulrichi stock.* 



Comparing D. prcenuntia and D. ulrichi it is evident that the two species are 

 closely related. In the tabulation of the zooecial tubes, which is the least variable 

 character in both, they are almost identical. The acanthopores furnish the only 

 reliable point of difference, these structures being much more abundant in D. ulrichi. 



Formation and locality. — The typical form and var. multipora are common in the middle third of the 

 Trenton shales at St. Paul, Minneapolis, and localities in Goodhue and Fillmore counties. Var. simplex 

 occurs in the lower and middle thirds at St. Paul and Minneapolis, while var. noBvigera is as yet known 

 only from Fillmore county, where it was found in the lower third of the shales. The var. echinata is rare 

 in the upper part of the middle third of the Trenton shales at Minneapolis, but abundant in the lower 

 part of the upper third at St. Paul, and near Fountain, Minnesota. 



*It may be well to add that the var. ecMnata will prohably prove to be the stock that produced Heterotrypa on the 

 'de and true Dekayia on the other. Also that the small DekayeUa obscura of the Cincinnati rocks may be a degen- 

 erate descendant of the var. multipora. 



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