366 ON THE CORALS OF THE 
in height. Its coralites, unlike those of the Liassic species, are 
very tall, extending in some specimens from the base to the 
upper surface. Its calices, however, are considerably smaller, 
and of more equal size, averaging about two lines across. ‘Their 
walls, which are thick and well-defined, stand up abruptly, with 
mural erectness, over the central depression, which varies from 
a line to a line and a-half in depth. They are divided by from 
twenty to twenty-four septa, of which, however, more than the 
one-half are rudimentary, leaving but from four to eight of 
their number to meet in the centre of the visceral cavity. In 
the thickness of its walls and the character of its septa, this 
Helmsdale Isastrea greatly resembles the /sastrea oblonga of 
the superior Oolite, —-a species which has been found hitherto 
only at Tilsbury, Wiltshire. It also resembles, however, though 
in aless degree, Jsastrea Richardsoni, —a coral of the Lower 
Oolite ; but it is possibly a new species. I have found in the 
same beds, though much more rarely, what seemed to be a 
different species of Isastrea, though closely allied to the one 
described. ‘The corallum, massive like that of the other, is 
always greatly smaller. The calices, however, are considera- 
bly larger, and rather thinner in the walls, which do not stand 
up so abruptly over the central hollows ; the septa vary from 
about twenty to twenty-four in number; and, where they meet 
in the centre, they rise in many of the calices into a protu- 
berant knob, like the termination of a true columella, which, 
however, like all the other species of the extinct genus Isastrea, 
it wants. A Thamnastrea is also found in the same beds, but 
always hitherto in a state of bad keeping. Unlike any of the 
Oolitic Thamnastrea figured by Messrs. Milne-Edwards and 
Haime, the corallum forms a mere incrustation on rocks and 
stones of older deposits than the Oolite, and is in some speci- 
mens less than half a line in thickness; the calices are small 
and shallow, and rather thickly set. The circular elevation, 
