EEV. A. E. EATON ON EECENT EPHEMERID.E OR MAYELIE.S. 125 



Abdomen of ordinary proportions ; segments 2-7 of nearly nniform length, shorter than 

 segment 8 (which is the longest) or 9 ; segment 10 short ; tlie postero-lateral angles of 

 the dorsum in segments 8 and 9 are acute and slightly prolonged ; forceps-basis entire, 

 the homologous lamina of the $ obtuse. Lobes of the penis without apparent stimuli. 

 Median caudal seta subequal to the others, which in both sexes are abou.t as long as the 

 body. Ungues in every tarsus dissimilar each to the other. Fore tarsus of 6 about 1^ 

 as long as the tibia, which is nearly twice as long as the femur ; its joints, in diminishing 

 order, rank 2 »& 3 subequal, 4>, 5, and 1. Fore tarsus of ? (excluding joint 1) about f 

 as long as the tibia and joint 1 comljined ; the femur about as long as the tarsus, whose 

 joints rank 2, 3, 5, 4. Hind tarsus (excluding joint 1) about y as long as the tibia and 

 joint 1 comljined ; its joints rank 5, 2, 3 subequal to L The iirst joint in these tarsi is 

 obsolescent. Numph latent under stones or at the roots of water-weeds, in streams and 

 rivers. Body broadest at the mesothorax ; head slightly narrower than the pronotum, 

 and in anterior view trilateral, with the vertex arched and the oral r(^gion truncate ; 

 antenntie inserted about midway between the anterior ocellus and the sides of the face; 

 that ocellus is smaller tlian the others ; oculi moderately distant from each other in 6 . 

 Pronotum transversely quadraugular, arched above, nearly straight at the sides, and 

 obtuse at the anterior lateral angles. Abdomen plump, slightly convex beneath, and 

 somewhat quadrangularly arched above in segments 2-9 ; pleura? dilated considerably 

 in segments 3-8, slightly concave above, fringed with clavate or spathulate hairs, and 

 contributing to form, with the steeply sloping sides of the dorsum, a hollow for the 

 lodgment of the tracheal brauchia; ; those of segments 2 and 3 are obliquely truncate at 

 their posterior angles, but the pleura? of segments 4-7 are there acuminately pointed, 

 and constitute a series of uncinate serratures on each side of the body ; the pleune of 

 segment 8, less largely developed than their predecessors, are posteriorly more acutely 

 pointed in 6 than in 2 ; those of segment 9 terminate behind each in a triangular point, 

 which is perpetuated in the imago. The angularity of the dorsal arch, above referred to, 

 is due to longitudinal series of protuberances, ridges, or tubercles, one on each side of 

 the middle of the back, extending from segments 2-9 ; in segments 2 and 3 each promi- 

 nence is surmounted by an acute conical tubercle ; in segments 4-7 each ridge terminates 

 behind in an unciform tubercle pointing towards the tails ; in segments 8 and 9 the 

 ridges end abruptly. Abdomen broadest in segment 4 or 5, narrower posteriorly than 

 in front ; a line drawn touching the outer edges of the pleurte on each side would describe 

 a curve. Tracheal branchia; are borne by segments 3-7, and diminish in size successively 

 from the foremost ; those of segment 7 are completely obtected by the preceding pair. 

 The foremost lamina; are broad and obliquely quadrilateral, with^he corners obtuse or 

 rounded off, and have their greatest extension between the lower anterior and the upper 

 posterior corners ; the margin below the latter of these is slightly refuse ; the trachea 

 enters the lamina near the former. The hindermost of the tracheal branchia; have ovate 

 laminie, auricled obtusely at the base on the lower side. The other branchiae exhibit 

 gradations of form intermediate between these. Caudal sette nearly | as long as the 

 body; for some distance from the roots only their joinings are setulose and their joints 

 nude ; afterwards, until shortly before their extremities, the joinings are beset with 



SECOND SERIES. — ZOOLOGY, VOL. III. 17 



