180 Cincinnati Society of Natural History. 



Zoarium an undulating flabelliform expansion, attaining a 

 diameter of 5 cm. or more, consisting of irregularly inoscula- 

 ting slender subcylindrical branches, varying in width from 0.3 

 to 0.6 mm., but averaging about 0.45 mm. Fenestrules large, 

 subacutely elliptical, generally two or three times longer than wide, 

 but varying considerably in both dimensions; measuring longitu- 

 dinally, the average number in 1 cm. is between five and six • 

 transversely, nine or ten in the same space. These measurements 

 are obtained from the Tennessee specimens. In the examples 

 from Minnesota, which I refer here, the fenestrules are smaller, 

 averaging between six and seven in 1 cm. longitudinally. 



Reverse of the Tennessee specimens strongly rounded, nearly 

 smooth, or with faint longitudinal striae. In very young examples 

 the latter would probably be more distinct. Fig. b represents an 

 enlargement of the reverse of a small fragment obtained from near 

 the bottom of the Mississippi River gorge, at Minneapolis, Minn., 

 by splitting open a block of limestone. As usual under such con- 

 ditions the outer layer of sclerenchyma has adhered to the opposite 

 side of the matrix and exposed a more youthful stage in the devel- 

 opment of the zoarium, when the reverse side was strongly 

 striated. 



Obverse face very strongly convex, carrying three rows of 

 zocecial apertures. These are subcircular, with a scarcely appre- 

 ciable peristome, 0.09 mm. in diameter, and twenty-three to 

 twenty-five in 5 mm. The interspaces are generally depressed, 

 or form distinct pits between the ends of the cells. Sometimes 

 the apertures are arranged between faintly appreciable longitudinal 

 raised lines. Small acanthopores are usually scattered abundantly 

 between the zocecial apertures. 



Thin sections show that the long tubular primitive portion of 

 the zooecia, which is moderately long and prismatic, is often inter- 

 sected by from one to three diaphragms. Just before bending 

 outward to open at the surface the tubes become rounded, leaving 

 irregularly shaped interspaces or shallow mesopores. It is here 

 also that the acanthopores are developed. 



This fine species stands in an intermediate position between P. 

 trentonensis, Nicholson, sp., and P. grani striata, Ul. The first is 

 more robust, has the rows of cells more numerous and the axial 

 portion of the zooecia much longer. The second has more rigid 

 branches, longer and narrower tenestrules, and the reverse of the 



