486 Miscellaneous. 



the Coal-measure and Cretaceous shells before mentioned have been, 

 even the most massive Unio.s would doubtless be found with as thin 

 a shell as Mycdina and Inocenimus now possess. 



Mr. F. B. Meek's iiivestig-ations have shown that the prismatic 

 structure is a very common (if not a constant) character of the fossil 

 AvicuHdiB ; and it is doubtless of much value as a family character ; 

 but since it is also seen in certain genera of Mytilidae and the 

 Naiades, it is known that it is not the peculiar property of any 

 family. — Silliman's American Journal, May 1868. 



Smelts breeding in an Aquarium. 



Mr. Brightwell, passing through the Norwich fish-market the 

 other day, had his attention called by a man to his aqiiarium, 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 Cael Semper. 



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 harrier reefs (that is to say, such as fringe elevated 

 islands lying in the sea) and the coast reefs occurring in aU 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 

 harrier 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 north 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. 



