256 THE PALEONTOLOGY OF MINNESOTA. 



[Aspidopora . 



3 ram. Of these the average diameter is 0.2 mm. or a fraction more, but that of the 

 largest in the clusters is commonly about 0.3 mm., though of a few it may by 0.35 

 mm. Mesopores numerous, readily distinguishable even at the surface, not surround- 

 ing the zooecia entirely. Acanthopores very small and inconspicuous. 



Internal characters: These are faithfully and amply illustrated on plate XVII 

 rendering a description unnecessary. 



In my original description of the species I included another which is now sepa- 

 rated as Mesotrypa ( ?) spinosa. While I still regard them as related forms, the result 

 of a study of larger and better collections is to show that the latter is more inti- 

 mately connected with Mesotrypa infida than with the form to which I propose to 

 restrict the use of the name A. parasitica. I found no difficulty in distinguishing 

 specimens of the two species after discovering that they were really distinct. In 

 the parasitica the thickness of the zoarium is constantly 0.5 mm. or less, the acan- 

 thopores very inconspicuous, and the mesopores readily determinable under a good 

 lens. In the spinosa, on the contrary, the zoarium may attain a thickness of over 

 2 mm., the mesopores are scarcely distinguishable at the surface, and the acantho- 

 pores strong. Among associated parasitic species there is another that the beginner 

 may find it troublesome to separate. This is the Atactoporella insueta, really a very 

 different species, with more numerous acanthopores and less regularly distributed 

 zooecial apertures. 



Formation and locality. Kather rare in the lower and middle thirds of the Trenton shales at Minne- 

 apolis, St. Paul and Fountain, Minnesota. 



Mus. Reg. Nos. 5994, 5995. 



ASPIDOPORA ELEGANTULA, n. sp. 



PLATE XVII, FIGS. 13-21. 



Zoarium a thin, concavo-convex expansion, 25 to 38 mm. in diameter, and 0.5 

 mm. or less in thickness; sometimes consisting of two or more of such layers grown 

 loosely over each other. Under surface concave, with a concentrically and radially 

 marked epithecal membrane. Upper or convex surface celluliferous. Zooecial aper- 

 tures ovate, their walls thin, in contact only at limited points, the interspaces 

 distinctly occupied by mesopores, all the parts nicely arranged around clusters of 

 large cells. The latter are 3 or 4 mm. apart, and in most cases slightly elevated. 

 Average size of ordinary zooecial apertures about 0.18 mm, by 0.25 mm., with eleven 

 or twelve in 3 mm.; size of the largest in the clusters rarely exceeding 0.28 mm. by 

 0.37 mm. Acanthopores wanting. 



