54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



DESCRIPTIONS 



But little need be said in preface to the following descriptions of 

 genera and species. In addition to the treatment of new forms, 

 there is given considerable supplementary descriptive matter on 

 forms already described. 



As a basis of operations in determining the North American Mus- 

 coidea, the recent Smithsonian Catalogue of North American Dip- 

 tera, by Professor Aldrich, will be found quite indispensable. Its 

 value lies in its references to descriptions. It will be necessary to 

 use it with much caution so far as the synonymy is concerned. It 

 should also be pointed out that the sequence of genera there em- 

 ployed is unnatural and misleading. This is not the fault of the 

 cataloguer, but is due to the present unsatisfactory state of the litera- 

 ture of the North American forms. 



The sequence of subfamilies and tribes here adopted is as nearly a 

 natural one as is possible of attainment in the present state of our 

 knowledge. No doubt further study will modify this arrangement 

 in certain details. 



It is to be noted that the tribes which appear in center heads are 

 independent of the subfamilies preceding them, except those in 

 italics under the families Muscidae and Phasiida?. 



Family MACRONYCHIIDiE 



Tribe Trixodini 



Genus Trixodes Coquillett 



Trixodes Coquillett clearly exhibits in its weakly developed mouth- 

 parts, peculiar facial plate, and weak macroehaeta; a close affinity 

 with the CEstridse. The type species is obesa Coquillett, described 

 from a specimen collected by the writer in the Sierra Madre of Chi- 

 huahua. A second specimen was collected by the writer on the 

 West Fork of the Gila, in New Mexico. 



Subfamily Megaprosopix.k 

 Genus Microphthalma Egger. 



Microphthalmos trifasciata Say. — Tachina disjuncta Wiedemann 

 may be a small specimen of this species. 



The genus Microphthalma is distinct from Dexiosoma, from which 

 it differs in its relativelv small eves, almost bare and much shortened 



