1914] 



287 



dorsal-plate lias two large and conspicuous (subtriangular) lateral 

 patches of yellow colour, and the segments following, instead of a 

 mere " marginal belting " of that colour, have practically their whole 

 sides yellow right np to the base of each ring. The venter also is 

 much more largely yellow than black, some segments are yellow right 

 across, and all are widely so at their sides at least (never merely 

 spotted with yellow !) 



After considering the claims of all the black and yellow British 

 Fossors yet described, I have come to the conclusion that there is one, 

 and only one, in which I can recognise every single character ascribed 

 by Forster to his xanthocephala— viz., the <$ of the common and well- 

 known Cerceris arenaria, F. Two possible objections to this identi- 

 fication ought perhaps to be met before accepting it, but both in my 

 opinion can be met quite completely. 



1. It may be asked — since Forster was undoubtedly familiar 

 with Linne's descriptions of arenaria in " Sy sterna Nature " and 

 " Fauna Suecica," why did he fail to recognise the species when he 

 met with it ? 



But, if these descriptions be consulted, it will be found that the 

 characters given in them are those of the $ . Although the two sexes 

 of arenaria are very similar, they differ in points, which in Forster's 

 day might quite well have been thought to indicate specific difference, 

 e.g., arenaria ? has generally yellow markings on the basal segment, 

 and on its face — instead of the ( <$ ) " macula magna flava posticetri- 

 fida " described by Forster — it has as Linne states in Faun. Suec. 

 " three yellow spots." 



2. It has been suggested to me that Forster's expression abdo- 

 mine subsessili does not well apply to a Cerceris, since in that genus 

 the basal segment is more or less coarctate and might be called 

 a petiole. 



But abdomine subsessili is a Linnean phrase, and Forster no doubt 

 uses it in the Linnean sense. Linne in Sy sterna Naturae divides all the 

 species of Sphex into two groups only, viz. (1) " abdomine pet iolato, 

 petiolo elongato, and (2) " abdomine subsessili, ,, or, as we might say, 

 (1) Long-waisted forms, and (2) Short- waisted forms. In the first 

 division he places species now reckoned under Sphex, Scelip>hron, 

 Trypoxylon, &c, where the abdomen is evidently " stalked " ; in the 

 second all other Fossors, Cerceris included, e.g., arenaria, the very 

 species now under consideration. 



Obviously it is prima facie more likely that Forster should have 



