30 
24, Perrata Loaant (Nicholson). 
(Plate III. Figs. 3-3 f.) 
Petraia Logani,. (Nicholson), “ Canadian Naturalist,” Vor VIL, Naw aves 
Corallum small, turbinate, more or less curved, almost trigonal in transverse section, 
_owing to its being flattened on the side of the convex curvature, and also on the lateral sur- 
faces. Septa twenty- -six or twenty-eight a little above the base, but sixty or more at the cup, 
the increase of number being due to the bifurcation of each primary septum at a distance 
about a line and a half above the base, and also to the intercalation of new septa along both 
sides of a line which runs along the dorsal or convex side of the coral from top to bottom. 
This line is marked on the exterior by two primary septa, which form a prominent ridge exter- 
nally and pass inwards to the centre of the coral. At the margin of the cup the septa are 
somewhat unequally developed, being alternately larger and smaller, the larger primary septa 
‘being prolonved inwards to the centre of the coral, where they become somewhat bent and 
twisted together, No columella appears to be present, nor are there any fabule. The flat 
sides of the septa are furrrowed with a succession of deep grooves, about four or five in the 
space of one line, which are directed in an obliquely ascending and arching manner from the 
wall towards the centre, the interspaces between them being tumid and rounded, and thus 
imparting a crenulated appearance to the outer edges of the septa when exposed. These arch- 
ing grooves are not connected with lamellar dissepiments having a similar direction, but the 
septa for some little distance below the cup are united by delicate ‘transverse dissepiments. The 
epitheca is marked with a few annulations of growth, which are mostly very obscure, and with 
well marked costze corresponding with the septa. 
In none of the specimens in my possession does the epitheca extend more than half an 
inch (often less) above the base of the corallum. Beyond this point to the margin of the calice 
the edges of the septa are seen with their characteristic crenulated appearance, and united 
here and there by minute dissepiments. As already noted, the flattened convex side of the 
coral always exhibits two pre-eminently large septa produced by the splitting of one, which 
run from the top to the bottom of the coral in a straight line. The remaining septa are 
directed obliquely from both sides towards this central pair ; so that new septa are interca- 
lated along this line in proceeding from the base to the calice. It is probable that these two 
septa may mark the position of asfossule in the cup, but none of my specimens exhibit the 
interior of the calice, and I am, therefore, unable to speak positively on this point. For 
the same reason I can say nothing as to the condition of the free edges of the septa inter- 
nally. 
The total length of the corallum is from three-quarters of an inch to one inch, the diameter 
of the cup varying from half an inch to nearly three-quarters. The calice is oblique, so that 
the greatest length of the coral is along its convex curvature, 
Petraia Logani is closely allied to Petraia (Turbinolopsis) pluriradialis (Phillips), with 
which I was at first sight disposed to identify it. It is, however, readily distinguished by the 
flattening of the convex curvature and lateral aspects of the coral, and by the smaller number 
of radiating septa. As regards other more minute characters, the published descriptions of 
P. pluriradialis are not sufficient to enable any closer comparison to be instituted with advant- 
age between the two species. 
With regard to Heliophyllum exiguum (Billings), it may be well to present, in a summary 
form, the points of agreement and difference which appear to exist between the two species :— 
1. Both corals are of the same general form and size, and both occur in the same forma- 
tion and at the same locality. 
2. Both possess a principal pair of septa, which are visible externally as two straight 
ridges extending from the top to the bottom of the coral, the remaining septa being directed 
obliquely towards this line on both sides. (When writing my original description of P. Logani 
in the Canadian Naturalist, 1873, I had not succeeded in “detecting this feature in H. exiguum.) 
3. The number of septa in the cup appears to be about the same in both, though stated 
to be sometimes as many as eighty in //. exiguum, whilst they never appear to exceed sixty- 
five in P. Logani. 
The above are the chief points of agreement, but there are the following points of differ- 
ence :— 
