FAUNA OF THE TRENTON GROUP. 
13 
fifteen pairs each. Median covering-plates have not been seen. An 
appearance of unusual width is given to the rays by the fact that the 
plates of the inter-radii which abut against them are somewhat 
higher than the remainder of those in the inter-radial spaces. 
The supra-oral area is large, and covered by numerous small 
plaitee. Their arrangement is difficult to make out, the inter-ambula- 
crals being peculiarly mixed with ray and supra-oral series. In the 
centre of the disk there appears to be a central plate dove-tailing into 
two plates that are between rays I and Y, and abut on the posterior 
inter-radius. At the sides and in front of the central piece are five 
more small plates, one on each side and three in front. Two are 
inter-radial in position, one between rays II and III, and one between 
III and IV. This is on the type, but on the small specimen next to it 
in the figure, there seem to be only five plates which really belong to 
the supra-oral series, the central, two posteriors, and two anterior 
laterals, bet ween rays II and m, and III and Y. 
The inter-radial areas are covered with small imbricating plates, 
those of the inner part of the outer marginal band being somewhat 
larger and wider than those between the rays. The posterior inter- 
radius is wider than the others, and the anal pyramid is large and 
distinct. 'Situated a little more than half-way from the centre to the 
margin, it is composed of a ring of seven or eight, long, triangular 
plates. In .some specimens it is observed half-way between rays I 
and Y, in others it is eccentric, and nearer Y than I, as in the type. 
The holotype is a large individual, 15*5 mm. in diameter. Other 
specimens on the same slab with it (all figured) are 11*5, 10, and 
9 mm. respectively. 
This species differs from Lebetodiscus diclcsoni in having shorter 
rays, one or two of which are solar, and in having much smaller inter- 
radial plates. It more nearly resembles Carney ella platys , but has 
more supra-oral plates. Though small specimens of Isoropbus incon- 
ditus have straight, broad arms, they may readily be distinguished 
from Carneyella billingsi or C. youngi by the greater number of 
supra-oral plates. 
Horizon and Locality. This species is common in the Cystid 
beds in the Prasopora zone on both sides of Ottawa river at Ottawa, 
and Hull. It occurs at Peterborough also. 
The holotype (No. 1409 Victoria Museum) was collected by Mr. 
T. C. Weston and is undoubtedly from Queens wharf. 
