BRYOZOA. 249 



Prasopora contigua. | 



PKASOPORA CONTIGUA Ulrich. 



PLATE XVI. FIGS. 24-26 



Prasopora contigua ULKICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 87. 



Zoarium hemispheric, base flat or slightly concave, usually less than '30 mm. in 

 diameter. Zocecia with very thin walls and polygonal apertures, ten or eleven of 

 the average size in 3 mm. Clusters of zocecia, some of them attaining a diameter 

 of 0.37 mm., occur at intervals of a little less than 4 mm. Mesopores comparatively 

 few, often difficult to detect at the surface. 



Internal structure: Tangential sections show that the zooecial walls are polygonal 

 and very thin, with neighboring cells in contact, except at many of the angles of 

 junction, these being occupied by one or two small mesopores. The latter often 

 form very inconspicuous clusters at the center of the groups of large zocecia, but 

 in the intermediate spaces not over half of the angles of junction between the 

 ordinary zocecia are occupied by mesopores. A few very small acanthopores are 

 developed. The opening left by the cystiphragms is generally of ovate form and 

 more often eccentric than central in its position within the tube cavity. 



Vertical sections are peculiar chiefly because they exhibit a marked decrease in 

 the number of mesopores when compared with other species of the genus. 



This form is closely related to P. simulatrix, and perhaps should be classed as a 

 variety of that species. The distinguishing features are (1) the much smaller num- 

 ber of mesopores, (2) thinner walls, (3) the presence of small acanthopores, and (4) a 

 somewhat greater crowding of the cystiphragms, these averaging over twenty in 

 2 mm. More specimens are necessary before the constancy, and, therefore, value, of 

 these differences can be established fully. The same species, very slightly modified, 

 occurs at Cincinnati, Ohio, about three hundred feet above the Ohio river bed. 



Formation and locality. The Minnesota specimens are believed to have been found either in the 

 upper part of the Trenton shales or in the Galena shales, at localities in Goodhue and Dakota counties. 



Must. Reg. Nos. 5301. 5534, 5989. 



PRASOPORA CONOIDEA Ulrich. 



PLATE XVI. FIGS. 11-15. 



Prasopora conoidea ULRICH 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 87. 



Zoarium generally depressed-conical, with the hight a little more than half the 

 diameter ; at other times subhemispherical ; hight varying from 4 to 16 mm., the 

 diameter from 8 to 20 mm. Under surface rather deeply concave, wrinkled concen- 

 trically, with a small central scar. Upper surface celluliferous and presenting, at 



