ORDOVICIAN FAUNAS. 137 
steep sides and a convex floor; its depth, measured from the centre 
of its floor, is equal to rather less than one-third of the height of 
the corallum. Septa well developed, of two orders—primaries and 
secondaries—alternating, numbering in all mature individuals from 
ninety to one hundred and twenty, the primaries reaching the centre, 
with their inner ends twisted, the secondaries extending but a short 
distance inward from the wall. The free edges of the septa in the 
calyx are denticulated. Dissepiments at times having the appearance, 
in longitudinal sections of the corallum, of true tabule, irregular, 
subordinate to the septa between which they curve upwards convexly 
toward the centre of the visceral chamber, where they lose their 
individuality, and form, with the twisted inner ends of the primary 
septa, an axial area of cellulose structure.”—Lambe. 
Remarks.—This species occurs more or less abundantly in some 
strata of the Trenton limestone, but usually the specimens are in a 
poor condition of preservation. It is the only species of horn coral 
which has been recognized at this horizon. 
NYCTOPORA BILLINGSI Nich. 
Plate VIIL., Figs. 1-2. 
1879. Nyctopora billingsi Nicholson, Pal. Tab. Corals, p. 184, pl. 
9, figs. 3-3 c. 
1899. Nyctopora billingsi Lambe, Cont. Can. Pal., vol. IV., pt. L, 
p- 49, pl. 2, figs. 1-1 a. 
Description.—Corallum depressed hemispherical, spheroidal or pyri- 
form, attaining a diameter of 40 to 50 mm. Corallites polygonal, 1 
to 1.25 mm. in diameter, their walls of moderate thickness, in close 
contact and entirely coalesced. The septa rather thick, projecting 
but slightly into the cavity of the corallite, eight to twelve in number. 
Tabule horizontal, complete, two or three in the space of 1 mm. 
Mural pores minute, circular, arranged in no definite order. 
Remarks—The genus Nyctopora differs only from Columnaria in 
the presence of mural pores, and only one species of the genus has 
been described. This species most closely resembles Columnaria halli 
Nich. in the character of its septa, but the individual corallites are 
