EEV. A. E. EATON ON EECENT EPHEMERID^ OE ilATFLIES. 149 



Section 8 of the Genera. — Type of Prosopistoma. Adult. — Similar to Ccenis and its 

 allies, bvit with 4 wings instead of 2 only. Figured by Vayssiere under very dis- 

 advantageous circumstances ; the figures consequently do not admit of exact comparison. 

 Nymph. — Fore wings immersed in a notal shield, which conceals the tracheal branchiae 

 and most of the abdomen : pronotum undefined. Palpus of maxilla i. 4-jointed, longer 

 than the lacinia ; the latter nude on the crown, armed with a few strong spinous teeth 

 at the tip and a few setulte on its inner side. Labium not differentiated into lobes and 

 lacinise of second maxillae. Hinder lateral angles of abdominal segments dilated and 

 produced. Natation agile, effected by the setae exclusively (which are capable of com- 

 plete retraction into the interior of the abdomen), the legs being closely folded up. 



PROSOPISTOMA, Lat. 1833. 



Illustrations. Suhimago (details), PI. XV. 27 (after Vayssiere, 1881), Nymph, 

 PL XLIII. ; see also citations under Binocle, Geoffrey (1764), Frosopistoma, Joly (1872, 

 Sept., and 1876 Mars), Westwood (1877), and Vayssiere (1881). 



Suhimago (in alcohol). — Wings 4; hind wings with the costal shoulder placed close to 

 the wing-roots ; neuration in both wings plentiful, but no cross veinlets are delineated. 

 Hinder ocelli relatively much smaller than in Ccenis. Abdomen proportioned somewhat 

 as in Ccenis, with the pleurfe of segments 7 -9 similarly produced into slender points ; the 

 ventral lobe of ? segment 9 entire and truncate. Caudal setas of ? ^ as long as the 

 body. The recurrent membrane of the fore wing-roots does not extend beyond the point 

 of the scutellum. S and adult fly unknown. Proportions of legs not ascertained 

 (after Vayssiere, 1881). Nymph. — Broadly ovate, tapering posteriorly, flattened beneath, 

 and highly convex above. Notal shield imperfectly peltate, being excised in front and 

 behind to fit with extreme exactitude the adjoining surfaces of the head and 7th abdo- 

 minal segment ; laterally its borders are broadly expanded, flattened beneath, and trun- 

 cate obliquely in front and behind ; dorsally, along the median suture, a narrow depression 

 or shallow furrow is apt to be produced, the integument thereabout being apparently of 

 a texture sufficiently yielding and elastic to allow considerable variation in the defini- 

 tion of the furrow. Possibly this part may be concerned in some way with the trans- 

 fusion of water through the branchial chamber underlying the shield ; but this is merely 

 my conjecture. At the hinder extremity of the median suture, a small aperture is tlis- 

 cernible at the edge of the shield, affording an exit from the branchial chamber. The 

 plastron (so to speak) truncate in front and behind, and nari'owed in advance of the 

 metasternum, is slightly countersunk in relation to the sternum and traversed by shallow 

 grooves for the lodgment of the legs when they are folded up during adhesion or nata- 

 tion. The sutures of the mesosternum, with the pro- and meta-sterna, are liable to 

 become effaced in alcoholic specimens ; neither the artist nor myself could distinguish 

 them in the subject of PI. XLIII. The sternum terminates behind in an acuminate point, 

 very near the margin of the plastron, and has a very smooth flattened surface. A narrow 

 ovate aperture exists on each side of the plastron close to the acute hinder angles ; 



S'ECOND SERIES. — ZOOLOGY, VOL. III. 2Q 



