53 DIPTERA OF NORTH AMERICA. [PART IV. 



The relative position of the subcostal cross-vein and of the tip 

 of the auxiliary vein is somewhat different in the majority of the 

 species of both genera. In both of them, the cross-vein is some- 

 times placed at the tip of the auxiliary vein ; but whenever it is 

 removed from this tip, it is always situated between the first 

 longitudinal vein and the auxiliary in Dicranomyla; in Lim- 

 nobia, on the contrai-y, it is very often situated between the 

 auxiliary vein and the costa ; in which case, the auxiliary vein 

 ends in the first longitudinal vein, and not in the costa, as usual. 

 This latter structure, as far as I have observed, occurs only iu 

 the genus Limnobia. 



The relative position of the marginal cross-vein and of the tip 

 of the first longitudinal vein, gives occasion to a somewhat similar 

 observation. In Dicranomyia this cross-vein is always at the tip 

 of the first longitudinal vein (about the peculiar structure, some- 

 times occurring here, compare above, page 54). In Limnobia 

 the marginal cross-vein is often some distance back of this tip, so 

 as to cut the stigma in two, or even to be placed near its inner 

 end. This latter character, as far as my observation goes, may 

 be useful in doubtful cases, as it occurs principally in the less 

 typical Limnobiae, some of which, as for instance L. macrostigma, 

 might be mistaken for Dicranomyise. The typical Limnobise 

 (cincfipcs, soUtaria, etc.) have the cross-vein close at the tip of 

 the first longitudinal vein, like Dici^anomyia. 



We may also notice here, that the discal cell is often open in 

 the genus Dicranomyia, whereas I know of no such case among 

 the Limnobise. 



In ray previous essay on the Tipulidse brevipalpi, the genera 

 Bhipidia, Geranomyia, Dicranomyia, and Limnobia have been 

 united as subgenera of a single genus Limnobia. This was done on 

 the ground that Rhipidia and Geranomyia are much more closely 

 allied to Dicranomyia than the latter is to Limnobia (in the 

 narrower sense). If thei'efore we leave Limnobia and Dicra- 

 nomyia united, we should not separate Rhipidia and Geraywmyia 

 from them. If, on the contrary, we separate Rhipidia and Gera- 

 nomyia from Dicranomyia, we should, d fortiori, separate Dicra- 

 -nomyia from Limnobia. This separation has been introduced 

 in the present publication. 



The name Dicranomijia (from bix^avov, fork, and ^iia, fly) has 

 been first used in J. Stephens's Catalogue of British Insects, in 



