486 Miscellaneous. 
the Coal-measure and Cretaceous shells before mentioned have been, 
even the most massive Unios would doubtless be found with as thin 
a shell as Myalina and Inoceramus now possess. 
Mr. F. B. Meek’s investigations have shown that the prismatic 
structure is a very common (if not a constant) character of the fossil 
Aviculide ; and it is doubtless of much value as a family character ; 
but since it is also seen in certain genera of Mytilide and the 
Naiades, it is known that it is not the peculiar propesty of any 
family. ig udliman’s American a, May 1868. 
Smelts breeding in an Aquarium. 
Mr. Brightwell, passing through the Norwich isheisareet the 
other day, had his attention called by a man to his aquarium, in 
which he found some smelts, caught in the river, were kept alive, 
They had deposited spawn on the stones at the bottom; and the 
young fry had emerged, so exceedingly minute as scarcely to be 
seen, but distinguishable as young smelts. They make excellent 
microscopical objects.—L. B. 
On the Formation of Coral Reefs. By Cart Sumper. 
The well-known annular form of the reefs containing lagoons, the 
atolls, was formerly explained by supposing that the polypes had 
built their dwellings, perpendicularly upwards, upon the margins of 
the craters of submarine volcanos, by which an external ring (an 
outer reef) must necessarily be produced, closing the crater, now 
become a lake, against the outer sea. In this, however, the allied 
forms of the barrier reefs (that is to say, such as fringe elevated 
islands lying in the sea) and the coast reefs occurring in all tropical 
seas were not taken into consideration. Darwin, by his theory, 
brought the three forms into mutual connexion. He thought he 
could demonstrate that the atolls and barrier reefs could only be 
explained by the assumption of the gradual sinking of a continent or 
island, and the coast reefs by an elevation of the shores. Although 
he himself called attention to some difficulties, he believed he could 
support:-the value of his theory in opposition to such obstinate facts, 
especially by demonstrating how in general the coast reefs were 
formed only on shores now in course of elevation, the atolls and 
barrier reefs, on the contrary, in regions of the sea in which the 
want of all active volcanic energy indicates a depression. 
Nevertheless cases do occur which cannot be explained thus. 
Leaving out of consideration the Philippines, where several atolls 
‘are found in the midst of islands in course of elevation, the western 
Caroline Islands, the Pelew Islands, furnish a very striking example 
of an association of extreme forms. At the nares of the chain of 
islands (which stretches nearly north and south, and is about sixty 
geographical miles in length), there are true atolls ; in the middle, 
