250 THE PALEONTOLOGY OF MINNESOTA. 



[Prasopora selwyni. 



intervals of about 3 mm., more or less prominent monticules, whose summits usually 

 appear subsolid or minutely pitted ; their slopes are occupied by zoo3cia above the 

 average in size. Zooecial apertures subcircular, eleven or twelve of thpse of the 

 ordinary size in 3 mm. Mesopores abundant, in most cases readily distinguishable 

 at the surface with the aid of a good lens. Acanthopores small, inconspicuous. 



Internal characters: These require no detailed description, being brought out 

 sufficiently in figs. 14 and 15. Compared with those of P. simulatrix Ulrich, we find 

 that the zoo3cial walls are thinner, the mesopores rather more abundant, and that 

 small acanthopores, one or more to each zocecium, are present, these structures being 

 absent in P. simulatrix. The tabulation of both sets of tubes is also more compact, 

 the average number of diaphragms in the mesopores in 1 mm. being about thirty, 

 and the cystiphragms in the zooecial tubes over twenty-five in 2 mm. In the latter 

 respect the species is nearer P. contigua Ulrich, from which it is distinguished by its 

 smaller size, more conical form, tuberculated surface, and more numerous mesopores. 

 More than five hundred specimens show that the subconical form, more or less 

 developed monticules, the strongly concave base, and the small size of the zoarium 

 are persistent characters, sufficing to distinguish the species almost at a glance from 

 other forms of the genus. 



Formation and locality. Restricted to the upper third of the Trenton shales, occurring rather 

 rarely at St. Paul, but more abundantly at several localities in Goodhue county. The best locality is at 

 Oxford Mills, near Cannon Falls. 



Mus. Beg. Nos. 3483, 7622, 8024, 8037. 



PRASOPORA SELWYNI Nicholson. 



,_ PLATE XVI. PIGS. 16-17. 



Monticulipora (Diplotrypa) whiteavesii (part.) NICHOLSON, 1879. Pal. Tab. Corals, p. 316. 

 Monticulipora (Prasopora) selwynii NICHOLSON, 1881. "The Genus Monticulipora," p. 206. 



Zoarium discoid, subconical, or hemispheric, rarely more than 30 mm. high, and 

 in most cases varying between 40 and 90 mm. in diameter. Base flat or gently con- 

 cave, the epithecal plate striated or wrinkled concentrically. Surface characters of 

 the Minnesota specimens obliterated through weathering, the only one still distin- 

 guishable being the substellate maculae. Their specific characters, however, are 

 clearly determinable by means of thin sections. 



Tangential sections are not materially different from those of P. simulatrix 

 Ulrich. As a rule the zooecial walls are a little thinner, and the mesopores of larger 

 size. But vertical sections, as may be seen by comparing figs. 1 to 5 with 16 on plate 

 XVI, are quite different. The tabulation of the tubes is on the whole less compact, 



