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paratively stunted inhabitants of the Mediterranean and other seas 

 of less elevated temperature. 



In Operculina, on the other hand, we find the shell presenting 

 the minutely tubular structure which was first shown by the author 

 to exist in Nummulites ; to which genus Operculina is so closely 

 allied in structure, that the only positive difference between them 

 seems to lie in the tendency of Operculina to open out widely in the 

 last whorl, whilst Nummulites (according to MM. d'Archiac and 

 Haime) tends to close in. The author minutely describes the struc- 

 ture of Operculina, which presents a very remarkable development 

 of the canaliferous system ; he also enters into a detailed inquiry 

 into the relation of the numerous strongly-marked varieties of form 

 which it presents, a question of much importance in regard to the 

 value of the characters of the reputed species among Nummulites ; 

 and shows that the range of individual variation in form and surface- 

 markings is so wide (as is proved by the gradational transitions 

 which present themselves between what at first sight appear to be 

 widely-separated types), that only where some very decided and con- 

 stant difference of internal conformation presents itself, will it be safe 

 to assume a specific diversity. In one case, in which he had thought 

 that a certain series of specimens was sufficiently distinguished 

 by its peculiar physiognomy from the rest, residual forms presented 

 themselves which could not be with certainty assigned to either 

 type, so completely do they link together the two by the softening 

 down of the peculiarities of each. And a yet more remarkable link 

 of connexion is established by examples collected on the coast of 

 Japan by the American expedition to that country, in which the 

 most distinctive characters of each type are curiously combined. 



Closely related to Operculina is another genus, Amphistegina, 

 which bears an equally near resemblance to Nummulites, though it 

 has been completely separated from both in the classification of 

 M. d'Orbigny, whojias placed it in a distinct order, Entomostegues, 

 on account of the unsymmetrical form of its shell and the alternating 

 disposition of its chambers. But the author has found, from an 

 extensive comparison of individuals, that this want of symmetry is so 

 little constant, as to be altogether valueless in a systematic point of 

 view, many specimens being perfectly symmetrical, whilst others 

 are very far from being so, and every gradation presenting itself 



