4'96 Linnaan Society. 



endeavours thereby to account for the readiness with which some 

 siHcified woods break down into sepai*ate portions, exhibiting perfect 

 casts of the organs within which the siliceous matter was deposited, 

 while others «i|^ cemented into a mass incapable of organic sepa- 

 ration. 



Read also " Notes on the Variations of Structure in the British spe- 

 cies of EurytomidcB." By Francis Walker, Esq., F.L.S. &c. 



In this paper Mr. Walker enumerates the variations in each seg- 

 ment of the British Eurytomidce, and comes to the conclusion that, 

 in grouping tlie species of a genus of this family, the primai'y divi- 

 sions may be formed from the variations of the thorax, and tlie secon- 

 dary divisions from the variations of the abdomen, of the antennae, 

 and of the nervures of the wings. He regards Eurytoma as the ty- 

 j)ical genus of the family ; and believes that the three genera Isosoma, 

 Systole and Decatoma converge towards it by as many radii. An un- 

 described genus, to which j\Ir. Walker gives the name of Porc'ta, is 

 nearly allied to Decatoma, and is thereby connected with Eurytoma. 



Mr. Walker takes a summary view of the three genera Eurytoma, 

 Isosoma and Decatoma, noticing under each the peculiar characters 

 of the genus and the modifications to which they are subject. He 

 points out the number of variations which occur in the British spe- 

 cies in the structure of their segments, and gives arranged lists of 

 the species, commencing with those which are most characteristic of 

 the genus, and ending with those which are least so. 



Read also the conclusion of Mr. Doubleday's " Remarks on the 

 genus Argynnis of the ' Encyclopedic Methodique,' especially in re- 

 gard to its subdivision by means of characters drawn from the neu- 

 ration of the wings." 



Mr. Doubleday commences by referring to the successive attempts 

 made by Jones, by M. Boisduval, and by M. Lefevre to apply the 

 characters drawn from the neuration of the wings to the arrangement 

 of Lepidoptera ; and to the use of characters dei-ived from the same 

 source in the works of M. de Haan, Dr. Rambuhr, and Mr. West- 

 wood. In the present paper he endeavours to test the value of the 

 neuration of the wings in subdividing a large natural group, for which 

 purpose he selects the genus Argynnis of the ' Encyclopedic Metho- 

 dique.' 



After stating generally the theory of the wing proposed by M. 

 Lefevre, Mr. Doubleday proposes an amended theory as follows : 

 " That the structure of the wings in insects is to have two distinct 

 sets of air-vessels or nervures, three belonging to the anterior half of 

 the wing, three to the posterior ; that in those species in which the 

 wings are in the most truly normal condition these nervures are all 

 fuUy developed and all subserve to their true functions ; that in de- 

 scending from these we first find some of the nervures less developed, 

 but still subser^^ng to their functions, then becoming gradually atro- 

 phied, and at last disappearing altogether ; and that this gradation de- 

 pends parth' on the rank which the species hold in the true system of 

 nature, and partly on their oeconomy." The three upper nervures exist, 



