160 THE PALEONTOLOGY OP MINNESOTA. 



LTrigonodictya. 



is no real affinity between them, this being, as is clearly shown by transverse sec- 



I 

 tions, a bifoliate zoarium, while in Nematopora the zooecia diverge equally in all 



directions from the center of the branch. I am not acquainted with any species of 



Pachydictya, nor with any associated species of bryozoan, with which the slender 



ramulets of P. triserialis might be confounded. 



Formation and locality. — As yet known only from the Trenton limestone at Montreal, Canada, but 

 it is not at all unlikely that the species is to be found in the Minnesota equivalent of that horizon. 



Genus TRIGONODICTYA, n. gen. 



Zoaria with triangular branches, constructed upon the plan of Prismopora, but 

 with zooecia and all minute details of structure precisely as in Pachydictya. 



Type : Pachydictya conciliatrix Ulrich. 



Another species occurs in the 'Clinton rocks near Eaton, Ohio, which, because it 

 is the only bryozoan with triangular branches known to me from Upper Silurian 

 strata, and may therefore be distinguished from associated forms with ease, I pro- 

 pose to name Trigonodictya eatonensis, n. sp. It is rather more slender than the Tren- 

 ton species, and its branches divide at less frequent intervals. The three surfaces 

 are also flat instead of concave, while in thin sections the interspaces between the 

 comparatively large oval zooecia are thinner, and the lines of erect median tubuli 

 much less distinct and not so numerous. 



Trigonodictya conciliatrix Vlrich. 



PLATE IX, FIGS. 11 and 12; PLATE X, FIGS. 15-20. 



Pachydictya conciliatrix Ulrich, 1886. Fourteenth Ann. Eep. Geol. Nat. Hist. Surv. Minn., p. 16. 



Zoarium of irregular growth, dividing at frequent intervals, consisting of equal- 

 sided triangular branches, with the three faces concave, each averaging about 3 mm. 

 wide ; or of more or less rapidly spreading, small, flabellate fronds, with from one to 

 five salient, divaricating ridges on only one or both sides. All intermediate condi- 

 tions between these two extremes occur. Each of the surface ridges has a non- 

 poriferous, sharp summit, and, beginning as a mere line, it rises gradually until it is 

 sufficiently high to permit of the formation of a new triangular branch, when it 

 forms one of its edges. Zocecial apertures elliptical, slightly oblique, smallest and 

 arranged longitudinally over tlie central half of each face ; here with 12 or 13 in 5 

 mm., a faintly elevated line between the rows, and the width of the longitudinal and 

 lateral interspaces generally about equal to the respective diameters of the aper- 

 tures. Toward the non-poriferous edges the apertures are directed obliquely upward 

 and outward, and increase in size gradually till those in the outermost row are quite 



