252 THE PALEONTOLOGY OF MINNESOTA. 
[Prasopora oculata. 
Internal characters: The first peculiarity to be noticed in tangential sections is 
the relatively great abundance of the mesopores. In most specimens they form a 
complete ring around the zocecia, and it is chiefly the large cells in the clusters that 
are occasionally in contact at limited points. The zocecial walls are thin, and in 
. one section have the peculiar tubular structure shown in fig. 21. The cystiphragms 
are more numerous and extend to a less distance from the walls than in any other 
American species. The opening left by them is of various shapes, generally sub- 
angular, and often removed from the walls. True acanthopores have not been 
detected. ; 
In vertical sections the abundance of the mesopores, the narrowness of the 
cystiphragms, and the unusual crowding of the tabulation in both sets of tubes, 
are the distinctive features. In the mesopores the average number of diaphragms 
in 1 mm. is over twenty-five, while the cystiphragms may number as high as twenty 
in 1 mm., though the average is not likely to be over fifteen in that space. 
Figures 18 and 19 represent one of two examples that may prove distinct. In 
tangential sections it differs in the greater size of the mesopores, and in the different 
appearance of the cystiphragms. In vertical sections the cystiphragms are less 
crowded and extend farther inward, while the mesopores here and there change 
into vesicles, a peculiarity that has not been observed in the usual form of the 
species. Provisionally the doubtful form may be distinguished as var. filmorensis. 
This species is readily recognized by its small size and discoid shape. When 
studied by means of thin sections it cannot be confounded with any other known 
to me. 
Formation and locality.—This is probably the commonest and most characteristic fossil of the 
Galena shales, having been found in greater or less abundance at nearly every locality in the state 
where that horizon is known to be exposed. The species also occurs in the Galena at the quarries near 
Neenah, Wisconsin, and at Decorah, Iowa. Var. filmorensis seems to have come from a lower horizon at 
Fountain and Preston, both in Fillmore county. 
Mus. Reg. Nos. 310, 5991, 5992, 7569, 7601, 7611, 7628, 7637, 7644, 7666, 8057; var. filmorensis, 5990. 
PRASOPORA oOcULATA Ford. 
FIG. 15, c, d, PAGE 248. 
Prasopora oculata FooRD, 1883. Contr’. Micro-Pal. Cambro-Sil. Rocks, Can., p. 11. 
Zoarium a thin, discoidal expansion, 15 to 30 mm. in diameter, and from 1 to 3 
mm. in thickness. Of the Minnesota examples none is thicker than 2 mm., and all 
are nearly 25 mm. in diameter. Under surface flat or gently concave, and marked 
with more or less strong wrinkles of growth, and sometimes with very fine radiating 
lines. Upper surface celluliferous, exhibiting subsolid macule (clusters of mesopores) 
