558 PALAEONTOLOGY OF ILLINOIS. 



FENESTRAPORA OCCIDENTALIS Ulrich. 



PI. XLIV, tigs. 2 2a, and PI. LIV, flgs. 7-7e. 



Zoarium infundibuliform, expanding rapidly, almost salver- 

 shaped, but rounding gradually from the short stem into the 

 horizontal portion. The free margins are flat or somewhat un- 

 dulated. On the obverse or outer side, the branches appeal- 

 rigid, moderately strong, and increase in width from 0.3 to 0.5 

 mm.; fourteen or fifteen in 1 cm. Dissepiments short, depressed, 

 as wide or wider than the branches. Fenestrules rather regu- 

 larly oval, averaging 0.7 by 0.38 mm. with eight in 1 cm. Car- 

 ina about 0.4 mm. high, expanded at the summit, subangular 

 or rounded above; both sides present a series of large pits or 

 cells, separated by unequal intervals, but the average may be 

 placed at two in the space of a mm. One of the cells also oc- 

 curs on the top of the carina just beneath the bifurcations. 

 Zocecia in two ranges, one on each side of the median keel. 

 Apertures circular, nearly direct, about 0.09 mm. in diameter, 

 about twice their diameter apart, twenty in 5 mm.; and with 

 a faintly elevated peristome. 



On the reverse or inner side the fenestrules are broad oval, 

 being wider then on the opposite face; rather irregularly ar- 

 ranged, varying from quincuncial to directly opposite. In the 

 first order, which apparently prevails mainly near the center, 

 the branches are zigzag, and somewhat depressed below the 

 much stronger longitudinal interspaces that represent the dis- 

 sepiments. In the other arrangement the branches are nearly 

 or quite straight, and again much thinner than the rounded 

 dissepiments, which continue across them obliquely or at right 

 angles. The fenestrules are always wider than the branches. 

 Large cells, of the same nature as those in the carinae, with an 

 average diameter of 0.14 mm., and direct or oblique apertures, 

 are distributed over the surface of the branches and dissepi- 

 ments. Their number is about equal to that of the fenestrules. 



The internal structure is fully illustrated on PI. LIV. When 

 compared with F. biperforata Hall, from the same horizon 

 in New York, we find that that species is more delicate, has 

 the fenestrules smaller, the carina less pronounced, and the di- 



