EEV. A. E. EATON OX EECENT EPHEMERID.E Oil MAYFLIES. 277 



about 1^ its length. Vertex of 2 head transverse; posterior orbits of the oculi higher 

 than the interjacent portion of the edge of the occiput ; median ocellus prominent, 

 contiguous with the anterior margin of the upper surface of the head. Pronotum of 2 

 either refuse only or subcordately excised behind ; the reflexed lobes somewhat rounded 

 behind. 



Nymph {setiior). — Alulominal tracheal branchiae, when not spread, obliquely reclinate 

 at the sides, with Avell-developed suberect or ascending fascicled fibrils ; the laminae 

 successively increase in size from the first to the fifth pair, and then diminish gradually 

 to the seventh ; the fibrils attain their maximum develoj^ment at the third pair, and 

 decreasing successively from tliat point in opposite directions, both in density and 

 length, are lacking in the seventh pair ; the foremost lamina?, narrowly ovate-oblong in 

 form, are (like the next two) small in proportion to the fibrils ; those of the second 

 pair are ovate, of the fifth pair ovate-oblong, of the seventh obliquely obovate, and the 

 others exhibit intermediate gradations of form : branchial tracheae well defined, com- 

 monly alternate-pinnately branched, but occasionally (as in PL LXII. 20) arborescent. 

 Median caudal seta subequal to the others, minutely plumose, while they are unilaterally 

 so, and about as long as the body. Pronotum completely defined ; the lateral borders 

 broadly dilated, rounded ofi" in front, and produced behind into acute angles flanking the 

 anterior portion of the mesonotum. Ventral segments subequal in length ; the lobe of 

 9th segment of $ subtriangular ; pleurae narrowly dilated, saliently curved at the outer 

 edge anteriorly, and shortly acuminate behind, so as to form minute teeth. Pangs of 

 the mandibles narrowly subfalcate ; the exterior, and stronger, serrulate at the edges; 

 the interior, more sleudi?r and shorter, is in the left mandible or tridendate at the 

 point ; in the place of the endopodite, in both mandibles, is a short tuft of plumose 

 hair. Lacinia of maxilla i. beset with flattened pectinate teetli along the edge of the 

 crown, and terminated by a pair of contiguous flattened subulate si:)ines at tlie point : 

 first joint of the palpus moderately stout ; terminal joint slender, clavate, obliquely 

 truncate and densely velutinous at the end, pungent at the tip, and pubescent externally. 

 Median lolje of the tongue truncate-subcordate, with the au.ricles excised ; paraglossae 

 broadly unciform, Avith the points recurved. Pemora marked on the coloured side with 

 two irregular zigzag grey bands composed of confiueut sjiots, and apt to be respectively 

 connected with a spot near the base, and another, or two, near the tip. 



The nymphs figured respectively in PL LXI. and PL LXII. 24, probably are merely 

 junior grades of species of Ecch/io-tis; and the latter is the younger, judging from the 

 completeness of the delimitation of the pronotum at the hinder lateral angles. The 

 somewhat older N. American nymph (PL LXI.) ditl'ers from that of U. venosus of 

 senior grade in the following particulars. The lateral borders of the pronotum are not 

 produced posteriorly, but are intimately blended with the mesonotum ; and hence the 

 pronotum wants definition at the hinder lateral angles. The mandibles lack the tufts 

 of hair adjacent to the bases of the interior fangs ; and the left mandible has a single 

 slender bristle in their place, like that of those figured in Pis. LVIIL, LIX. The 

 median lobe of the tongue resembles somewhat that of the nymph last referred to; jjut 

 the paraglossae differ both from those of that nymph and from those of the senior 



