EEV, A. E. EATON ON EECENT EPHEMEEID.^ OR MAIFLIES. 235 



compressed femora protect the tracheal-brancliiaj ; but in swimming progression is aided 

 by scrambling movements of the legs. 



In the " Aid to the Gcnerical Identificatiou " of adult IMayflies of the Ecdyunis type, 

 Roman numerals designating tlie five joints of each tarsus stand at the head of the 

 columns of Arabic numerals which set fortli the proportionate lengths of the corresponding 

 tarsal joints of the species named in the left-hand column. The Aral)ic numerals, in fact, 

 state the millimetric admeasaremcnts of drawings diverse in scale of enlargement, made 

 with the aid of the camera lucida. The figures of the intermediate and hind tarsi are of 

 uniform scale with one another in the same species, but the scale is varied arbitrarily in 

 the different species inversely with the size of the specimens, so as to reduce the 

 dimensions of the longest joints approximately to a common standard of absolute length. 

 Similarly the figures of the fore tarsi are diversified in scale of enlargement with the size 

 and sex of the specimens, irrespectively of tlie figures of the hinder tarsi of the same 

 species, so as to facilitate the comparison of the proportions of the tarsal joints of the 

 fore leg in all specimens of either sex. 



Short Tabulation' of Nymphs of Section 1 i of the Genera. — Type of Ecdyurus. 



LaminjE of the foremost abdominal tracheal braiichia3 convergent, coming • 



into contact beneath the metasternnm ; brancliial fibrils 

 fasciculated. 



Median caudal seta present in specimens of advanced age Rhithrogena. PI. LIV. 



Median caudal seta entirely absent in aged specimens Iron. PI. LV. 



Laminse of the foremost abdominal tracheal branchiic widely remote from 

 each other. 

 Median caudal seta entirely absent in aged specimens. Branchial fibrils 



fasciculated Epeorns. PI. L VI. 



Median caudal seta present. 



LaminiK of the hindermost tracheal branchite conspicuously differ- 

 ent from the others, being liiiear-lanceolate instead of broad ; 

 branchial fibrils connate for some distance, thus forming a 

 single filiformly multifid membranous appendage to each of 

 the broader laminse, which are oblique at the base. 



Broader branchial laminae obovate-oblong and cuspidate Nameless Gen. PI. LVIl. 



Broader branchial lamina3 snb-quadrilatcral, mostly aurieled 



below at the base and rounded off at the other corners Nameless Gen. PI. LVIII. 



Laminje of the hindermost tracheal branchiaj very similar in form 

 to the others ; branchial fibrils fasciculated. 

 Branchial lamiufe of the intermediate pairs of tracheal 



branchiae mostly ovate-triangular and broad Nameless Gen. PI. LIX. 



Branchial laminie narrowly lanceolate Heptac/enia. PI. LX. 



Branchial lamiiue of the anterior pairs of the series ellipsoidal ; 

 those of hinder pairs pass gradually from oblong-oval to 

 obovate. In Junior specimens the laminaj are narrower 

 and trilateral Ecdyurus. Pis. LXT., LXII. 



