150 EEV. A. E. EATON ON RECENT EPHEIMERID^ OR MAYFLIES. 



through these inlets the water enters the branchial chamber. Head transverse, flattened 

 beneath, arched in front and above. Labrum small. Mentum oblong-oval, crenulated 

 behind, and with a truncate- triangular median excision in front, into which the labium 

 fits closely. The labrum and epistoma, with the labium and mentum, enclose the 

 remaining mouth-parts completely, and often conceal them. Antennae short, subulate, 

 6-jointed ; the joints in order of diminishing length rank 3, 4>, 5, 6 subequal to 2, and 1 ; 

 joint 1 is the stoutest, and joint 3 is nearly as long as all the others put together. 

 Mandible tapering from a l)road oblique subtriquetrous base to a pungent tridentate 

 crown ; fangs conical, the intermediate very little smaller than the others, just below the 

 bases of which the edges of the crown are minutely denticulated ; endopodite strong 

 and relatively long, subcyliudrical, bidentate at its extremity, with a row of denticula- 

 tions on each side just below the fangs ; its base is immediately preceded by a tuft of 

 velutinous or puberulose setae [their puberulence is not distinguishable in a figure drawn 

 to a scale of enlargement as low as 90], about 5 in number, and rather longer than the 

 endopodite ; molar region absent. The mandibles, as well as the 1st maxillae, are 

 virtually symmetrical ; the latter terminate each in 4 strong, flattened, acuminate, 

 chitinous teeth, the innermost of which are the strongest, and have 2 or 3 microscopically 

 puberulose seta? close to their inner base ; a short, solitary, smooth setula arises from the 

 inner face of the lacinia near the transverse suture ; the palpus, geniculated at the first 

 joining, has the proximal joint strongly reflexed ; its joints in sequence of lessening 

 length rank 3, 1, 2, 4 ; the first two are stout, the others slender. Labium truncate- 

 obtriangular, slightly rounded off at the corners, and bevelled at the sides to fit into the 

 gap in the mentum ; tongue and laciniae of 2nd maxillae absent ; palpi geniculated, 

 tapering distally, the proximal joints divaricate, and each nearly as long as the next 

 joint. The joinings of the anterior ventral segments are sometimes dimly discernible 

 through the plastron. Dr. Vayssicre describes and figures (1882, figs. 106 & 108) 5 pairs 

 of obtected tracheal branchiae ; his figures should be consulted. Caudal setae pliimose, 

 indistinctly articulated, and about f as long as the body. Legs slender ; the fore tibia, 

 in about ^ its length from the tip, is armed interiorly with a row of articulated spines, 

 denticulated on their inner sides. Hind leg rather the longest; the tarsus (claw 

 excluded) less than ^ as long as the tibia ; this last about f as long as the femur. 



Type. P. variegatum, Lat. 



Distribution. Rivers of continental Europe, and Madagascar. 



Etymolo(jy. -Kpoau-niov and (TTo^to, from the mouth-partS being well concealed by the 

 large mentum &c. as with a little mask. 



Prosopistoma foliacetjm, Fourcroy. Pis. XV. 27 [wings, after Vayssiere] & XLIII. 

 [nymph]. 



Le Binocle h queue en plumet [Hist, abreg. des Ins. dc Paris, ii. 660, pi. xsi. 3 e. f. g. (1764)] ; Geoff., 

 op. cit. ed ii. loc. cit. (1785) & ed. iii. (1799). 



Binoculus foliacens, Fourcroy, Ent. Paris, ii. 539 (1785). — B. pennigerus, Lat. Hist. Nat. Crust. & Ins. 

 iv. 122 (1802). — B. pisciforme, Dumeril, in Diet. So, Nat. iv. 106, Paris, Lenormant, art. Binocle (1816). 



