348

ON THE CORALS OF THE

form than for its bulk, measures full eighteen inches in length,by about a foot in height. Its coralites, unlike those of theLiasic species, are very tall, extending in some specimensfrom the base to the upper surface. Its calices, however, areconsiderably smaller, and of more equal size, averaging abouttwo lines across. Their walls, which are thick and well-de-fined, stand up abruptly, with mural erectness, over the cen-tral depression, which varies from a line to a line and a-halfin depth. They are divided by from twenty to twenty-foursepta, of which, however, more than the one-half are rudi-mentary, leaving but from four to eight of their number tomeet in the centre of the visceral cavity. In the thicknessof its walls and the character of its septa, this HelmsdaleIsastrea greatly resembles the Isastrea oblonga of the superiorOolite,—a species which has been found hitherto only at Tils-bury, 'Wiltshire. It also resembles, however, though in aless degree, Isastrea Richardsoni ,—a coral of the Lower Oolite ;but it is possibly a new species. I have found in the samebeds, though much more rarely, what seemed to be a differentspecies of Isastrea, though closely allied to the one described,The corallum, massive like that of the other, is always greatlysmaller. The calices, however, are considerably larger, andrather thinner in the walls, which do not stand up so abruptlyover the central hollows ; the septa vary from about twentyto twenty-four in number; and where they meet in the cen-tre, they rise in many of the calices into a protuberant knob,like the termination of a true columella, which, however, likeall the other species of the extinct genus Isastrea, it wants.A Thamnastrea is also found in the same beds, but alwayshitherto in a state of bad keeping. Unlike any of the Ooli-tic Thamnastrea figured by Messrs Milne-Edwards and Haime,the corallum forms a mere incrustation on rocks and stonesof older deposits than the Oolite, and is in some specimensless than half a line in thickness : the calices are small and