440 



NORTH AMERICAN INDEX FOSSILS. 



or nearly so; Sc close to border and extending to apex of wing; 

 R strong at the base, and extending parallel to Sc to the apex; 

 Rs very uniform throughout the family, its divisions are by sets of 

 threes, three sets of three each being the most typical; the first 

 division commonly somewhat in front of the middle line of the 

 wing, the upper of the three trifurcates, and later the lower of 

 this new set of three also trifurcates. The middle veins of these 

 sets of three are weak, lie on the folds, and appear like intercala- 

 tions; the attachment is either to the upper or the lower vein of 

 the group, or more rarely directly between them. M simple to or 

 beyond the middle of the wing, then breaking into a set of three 

 veins all of which remain simple. The interpolated vein lies in a 

 furrow, the outer branches and the media itself lie on folds. The 

 media, usually carrying the sector, is fused at the base with the 

 radius; Cu^ and Cu^ separate, just at their basal origin, each typi- 

 cally three-branched ; first anal strong with abrupt downward curve, 

 C short, a few mm. long, dividing, its stronger branch turning with 

 uniform curvature across the Sc, ending on R, and forming a 

 costal brace (Sellards) ; the weaker part turns toward and joins 

 the costal margin. Examples: Protereisma permianum Sellards 

 (Fig. 1745), Prodromus rectus Sellards, Scopus gracilis Sellards, 

 and others from the Permic of Kansas. 



Fig. 1745. Protereisma permianum Sellards, a Permic mayfly from Kansas, X 4> 

 showing head, thorax and first seven segments of abdomen, and wings partly restored. 

 (After Sellards.) 



