212 THE PALEONTOLOGY OF MINNESOTA. se 
[Phylloporina corticosa. 
“Obverse generally presenting a very irregular appearance. This is largely due 
to irregular, noncelluliferous deposits of sclerenchyma that occur at variable inter- 
vals. The fenestrules, however, also seem less regularly arranged than upon the 
reverse face. Surface of branches strongly convex, carrying from three to six or 
more rows of alternating and scarcely circular zocecial apertures. These are about 
0.09 mm. in diameter, without peristomes, and separated by intervals of less width 
generally than their diameter. Some of the interspaces are a little prominent. 
These may have contained acanthopores. Five or six cell apertures in 1 mm. 
~ “Although the preservation of the material is not the best for microscopical 
determination of internal characters, thin sections still bring to light the more 
salient features. They show that the zocecial tubes are intersected by numerous 
diaphragms ; that near their apertures they are still prismatic, resembling the zocecia 
of a Monticuliporoid, and that a few small cells, perhaps acanthopores, are scattered 
among the true zocecia. 
“This is an easily recognized species, being also quite distinct from all the 
others of the genus known. In its proportions it is somewhat like P. corticosa, from 
the same horizon, but they are not likely to be confounded, the strong carine on 
both sides of the branches in that species serving amply in distinguishing them.” 
Formation and locality.x—Rather rare in the upper third of the Trenton shales, at St. Paul, Minnesota. 
Puyxuoporina corticosa Ulrich. 
PLATE V, FIGS. 1-10. 
Phyllopora ? corticosa ULRICH, 1886. Fourteenth Ann. Rep. Geol. and Nat. Hist. Surv. of Minn., p. 61. 
Zoarium reticulate, arising from an expanded base, at first funnel-shaped and 
poriferous on the outer side, later on becoming irregularly undulating. Poriferous 
side presenting the appearance of a Fenestella or Semicoscinium with strongly 
carinate and more or less flexuous anastomosing branches, and much depressed 
dissepiments. Width of branches varying from 0.35 mm. to 0.9 mm., but averaging 
about0.5 or 0.6 mm.; thickness of branches and frond varying between 1.0 and 1.6 mm. 
Obverse: On each side of the sharp, and, apparently, spiniferous median ridge, 
there are rarely one, usually two, rows of irregularly alternating, circular peri- 
stomate zocecial apertures, with nine or ten in eachin 2 mm. As a rule, the dissepi- 
ment-like connections between the branches are short, depressed, and may or may 
not carry a small number of zocecial apertures distinct from the series belonging to 
the branches. Occasonally this division of the frond into rigid or flexuous branches 
and depressed dissepiment-like connections is not recognizable on all parts of the 
