212 THE PALEONTOLOGY OF MINNESOTA. 



[Pliylloporina 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 zooecial 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 zooecial tubes are intersected by numerous 

 diaphragms ; that near their apertures they are still prismatic, resembling the zooecia 

 of a Monticuliporoid, and that a few small cells, perhaps acanthopores, are scattered 

 among the true zoo3cia. 



"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 carinae on 

 both sides of the branches in that species serving amply in distinguishing them." 



Formation and locality. Rattier rare in the upper third of the Trenton shales, at St. Paul, Minnesota. 



PHYLLOPORINA CORTICOSA Ulrich. 



PLATE V, FIGS. 1-10. 



Phyllopora f corticosa ULRICH, 1886. Fourteenth Ann. Rep. Qeol. 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 

 aboutO.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 each in 2 mm. As a rule, the dissepi- 

 ment-like connections between the branches are short, depressed, an'd may or may 

 not carry a small number of zooecial 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 



