184 THE PALEONTOLOGY OF MINNESOTA. 



LStictoporella. 



equalling the diameter of the zooecial apertures. Arrangement variable, rather 

 irregular, generally in diagonally intersecting series, with from fifteen to eighteen, 

 but averaging sixteen in 5 mm. 



Internal structure, as shown in thin sections, very similar T^o that of 8. angularis, 

 the only differences noticed resulting from the much greater development of 

 mesopores. , 



The much greater abundance of mesopores distinguishes this species from 8. 

 angularis, var. intermedia, while the frohdescent habit of its zoariijm separates it 

 from all the other Lower Silurian species of the genus. Associated iii the same 

 strata are PachydictyafoUata and Phyllodidya frondosa ?, two bifoliate species likewise 

 characterized by foliaceous zoaria. In other respects, however, they differ so obvi- 

 ously from S. frondifera, that detailed comparisons are rendered unnecessary. 



Formation and tocai%.— Eather'^bundant in the lower beds of the Trenton shales, at Minneapolis, 

 St. Paul, Preston, near Fountain, and other localities in the state where this horizon is exposed. 



Mus. Reg. Nos. 5945, 594'7, 7650. 



Stictopokella ceibrosa Vlrich, 



PLATE X, FIGS. 21-25; PLATE XL FIGS. 22 and 23. 



Stictoporella^ eribrosa Uleich, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 69. 



Zoarium growing from a small expanded basal attachment into thin erect 

 fronds, perhaps 50 mm. in diameter, composed of branches that inosculate at short 

 intervals till there is produced a broad expansion perforated at more or less regular 

 intervals by circular or elliptical fenestrules. Both sides of frond celluliferous, 

 consisting of two equal layers of cells grown together back to back in the manner 

 'usual with bifoliate Bryozoa. Fenestrules elliptical, sometimes circular, varying 

 greatlyj the average size about 1.3 mm. by 1.8 mm., or less than the width of the 

 branches ; but the same frond may show extremes like' 1 mm. and less, and 3.5 mm. 

 Their arrangement is therefore more or less irregular, and taking other cribrose 

 species of the genus into consideration, this irregularity in the size and distribution 

 of the fenestrules is to be considered as a characteristic of the present species. 

 Width of branches generally 2.0 mm. or 2.5 mm. but varying between the extremes 

 of 1.5 mm. and 8.1 mm. Zooecial apertures small, subcircular or elliptical, the 

 average size about 0.1 mm. by 0.12 mm., set into sharply defined, polygonal or 

 rounded, sloping areas, about nine in 2.5 mm. Mesopores small, numerous, often 

 completely isolating the zooecia. Generally, however, the latter are in contact with 

 each other at limited points. Around the fenestrules there is a band, 0.5 mm. or 

 less wide, occupied solely by mesopores. As in other species of the genus the 



