[ 601 ] 



XLII. The Difference in the Number of Eyes with which Spiders are provided 

 proposed as the Basis of their Distribution into Tribes ; with Descriptions 

 of newly discovered Species, and the Characters of a new Family and three 

 new Genera of Spiders. By John Blackwall, Esq., F.L.S. 



Read June 18th, 1839; and April 21st, 1840. 



JMeSSRS. Walckenaer and Dufour have separated the Arachnida consti- 

 tuting the order Araneidea into two principal divisions or groups, the former 

 arachnologist having adopted a difference in the structure of the mandibles, 

 and the latter a variation in the number of the branchial sacs of those animals, 

 as the basis of their respective modes of distribution. 



M. Walckenaer's first group consists of spiders whose mandibles are arti- 

 culated horizontally, are prominent, and have a vertical movement ; the 

 species whose mandibles are articulated vertically, are not prominent, and 

 have a lateral movement, composing his second group. 



The characters supplied by the organic modification on which these groups 

 are founded appear to be too slight, and of too little importance in a physio- 

 logical point of view, to serve for the establishment of divisions of such mag- 

 nitude ; moreover, another objection to the value attached to these characters 

 by M. Walckenaer arises from the circumstance of their merging gradually 

 into each other, which prevents a clear line of demarcation being drawn 

 between them. 



An insuperable obstacle also presents itself to the adoption of M. Dufour's 

 distribution of the Araneidea into spiders with four and spiders with two 

 branchial sacs ; it being impossible to determine, even on dissection, to which 

 of the two groups numerous species belong. Indeed, that indefatigable and 

 profound naturalist, M. Latreille, in attempting to arrange the Araneidea in 

 accordance with M. Dufour's views, has fallen into the error of separating the 

 Ariadne insidiatrix of M. Savigny {Dysdera imidiatrix, Walck.) from the 



