SILURIAN FAUNAS. 21:9 
PRISMATOPHYLLUM INEQUALIS (Hall). - 
Plate XVIIL., Figs. 12-13. 
1852. Columnaria inequalis Hall, Pal. N. Y., vol. I1., p. 323, pl. 72, 
figs. 3 a—b, 4a-c. : 
Description.—Corallum growing in irregularly subhemispheric 
masses, composed of prismatic corallites, in contact on all sides, having 
five to eight sides when mature, but often with only three or four 
curved sides when immature. Each corallite has a calyx of moderate 
depth, and when full grown has thirty-six septa. In cross-section the 
septa are seen to be arranged more or less irregularly in four subequal 
groups, and between the septa are curved dissepiments, which, near 
the surface of the corallum, are restricted to a peripheral zone, sur-' 
rounding a central, circular portion of about one-third the diameter 
of the corallite. Deeper in the corallum the dissepiments also occupy 
the central portion of the cross-section, but are thinner and less 
abundant than in the outer zone. In vertical section, when cut directly 
through the middle of the corallite, the outer zone is seen to be filled 
with short, curved, vesicular dissepiments; in the central portion 
the dissepiments are much more elongate vertically. In a vertical 
section, cut a little to one side of the centre, the cross-sections of the 
septa, seen in the central vertical area, bear lateral spinules, which 
reach about one-half the distance across the interseptal locule. These 
lateral processes seem to be continuous horizontally across the inner 
portion of the septa. 
The masses of this coral frequently attain a diameter of 15 or 20 
em., and range in size from these dimensions to small colonies. The 
largest corallites have a diameter of 10 mm., but this size is excep- 
tional, and they range from this to the younger corallites only 1 or 2 
mm. in diameter near the initial points. “ 
Remarks.—This species is evidently a member of the genus 
Prismatophyllum as established by Simpson.* It is one of the com- 
monest forms in the coralline bed of the Decker Ferry formation. 
* Bull. N: Y¥. St. Mus., No. 39, vol. VIII., p. 238. 
