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 zooecia, and it is chiefly the large cells in the clusters that 

 are occasionally in contact at limited points. The zooecial 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 vei*tical 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 OCULATA Foord. 



FIG. 15. c, d, PAGE 248. 



Prasopora oculata FOORD, 1883. Contri. Micro-Pal. Cambro-Sil. HOCKS, 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 ram. 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 maculae (clusters of mesopores) 



