Rhinidictya.] 



BBYOZOA. 



135 



superior hemiseptum will distinguish the sections at once from those ot all other 

 species except R. minima. That species occurs at a higher horizon (Galena shales), 

 grew differently, has smaller elliptical zooecial apertures and much thicker, as well 

 as quite differently marked interspaces. 



Formation and locality. Rare in the lower third of the Trenton shales at Minneapolis, Minnesota. 

 Mus. Reg. Nos. 5936, 5937. 



RHINIDICTYA TRENTONENSIS Ulrich. 



PLATE VI, FIGS. 14-18; PLATE VII, FIGS. 6-9. 



Dicranopora trentonensis ULKICH, 1882. "Amer. Pal. Bry.,'' Jour. Cin. Soc, Nat. Hist., vol. v, 



p. 160, pi. 6, figs. 15, 15a. 

 Stictopora fldelis (part.) ULKICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 68. 



Zoarium branching dichotomously at intervals of from 8 to 20 mm. Branches 2 

 mm., or a little less, wide, sharp-edged, the non-poriferous margin very narrow 

 Zocecia in from eight to eleven ranges, nine or ten the commonest numbers. Aper- 

 tures nearly direct, comparatively large, of elliptical, subquadrate or hexagonal form, 

 with sixteen, rareiyseventeen,in 5 mm. longitudinally, and five in 1 mm. transversely; 

 those forming the marginal row usually a little larger than the average and directed 

 slightly outward. Interspaces thin, apparently without granules, the longitudinal 

 ones but little, if at all, elevated over those running transversely, the former gene- 

 rally a little zigzag in their course. 



In tangential sections dividing the zoarium just beneath the surface, the inter- 

 spaces are moderately thick, and contain a line of very minute pores running length- 

 wise between the rows of cells. Here the zocecia, or rather their " vestibular " por- 

 tions, are elongate-elliptical, but at a deeper level, where the section cuts down into 

 the primitive portion of the zoarium, they have the usual oblong-quadrate, or sub- 

 rhomboidal shape. In one of the sections showing this region (see pi. VI, fig. 18) 

 a row of "median tubuli" is distinctly visible in the transverse partitions. 



Vertical sections remind us much of R. nicholsoni and R. grandis, in this, that the 

 interspaces or walls are rather thin, and that there is not even a sign of a superior 

 hemiseptum at the base of the "vestibule," the walls being merely thickened a little 

 abruptly. In sections of thick examples a complete diaphragm may cross the tubes. 

 In such cases it is common to find each half of the zoarium, in part at least, to con- 

 sist of two superimposed layers of cells. 



A re-examination of the Tennessee type of this species has shown conclusively 

 that it is not a Dicranopora but a Rhinidictya, with relations to R. nicholsoni and R. 

 yrandis, its systematic position being nearly intermediate between them. From the 



