250 THE PALEONTOLOGY OF MINNESOTA. 
[Prasopora selwyni. 
intervals of about 8 mm., more or less prominent monticules, whose summits usually 
appear subsolid or minutely pitted ; their slopes are occupied by zocecia above the 
average in size. Zocecial apertures subcircular, eleven or twelve of those of the 
ordinary size in 3mm. 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 zocecial 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. simulatriz. 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 zocecial tubes over twenty-five in 2mm. 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. Reg. Nos. 3483, 7622, 8024, 8037. 
PRAsopora SELWyNI Nicholson. 
PLATE XVI, FIGS. 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- 
euishable being the substellate macule. Their specific characters, however, are 
clearly determinable by means of thin sections. 
Tangential sections are not materially different from those of P. simulatrix 
Ulrich. Asa rule the zocecial 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, 
