EEV. A. E. EATON ON EECENT EPHEMEEID.E OE MAYFLIES. 153 



Part III. Read February 10th, 1885. 

 Fourth Series of Group II. of the Genera of the Ephemeridce. 



Adult. — The anal (8) and second axillar (9-) ncrvures, together Avith the inner margin 

 of the mesothoracic wing, enclose a curved, trilateral, somewhat leg-of-mutton sliaped 

 space, which is abrupt at its anterior extremity ; anal nervurc distinctly separate from 

 the pobrachial (7) at the base of the wing; first axillar (9^) more or less curved, and 

 rather long ; second axillar approximated to or united with the first at the base, where 

 this is distinctly independent of the anal uervure ; prtebrachial nervure (C) essentially 

 simple. Hind wings small or absent ; costal border spurred or protuberant at about the 

 first f of the wing's length ; neuration incomplete, the subcosta somewhat curved. 

 Thoracic spiracles straight-lipped, usually closed in the dried insect ; mesothoracic 

 spiracle without a guard, its valves unequal ; when open, the apertures of both spiracles 

 are oval. Pronotum of ? closely appressed to the mesonotum. Hinder tarsi absolutely 

 4-jointed, with the proximal joint long. Forceps-limbs sessile upon the segment, with a 

 short fleshy protuberance from it between their proximal joints, which is not represented 

 in the ? . Penis normally retracted and invisible, very rarely protruded by captured 

 living specimens. Median caudal seta aborted. Oculi of 6 divided into two unequal 

 parts ; the upper segment, cylindrical or somewhat turbinate, is facetted solely on the 

 terminal surface ; tlie lower and much smaller segment, oval in form, is annexed to the 

 under (or rarely to the anterior) orbit of the former', and is facetted all over with facets 

 of less diameter than those of the turbinate part. Hinder ocelli large ; the foremost 

 much smaller. The virgin imago can live many days if kept in a moderately damp cool 

 place. Subimayo usually quiescent for at least twenty-four hours, standing upon all its 

 feet with wings erect and setae divergent. 



Section 9 of the Genera.— ly^a of Baetis. Adult .—Vionoiwm. of ? transverse and 

 short, prominent above and somewhat smooth, closely appressed to the mesonotum and 

 receding in the middle behind. Hind tibia at least | as long, and sometimes nearly of 

 the same length as the femur ; the tarsus from about I to about f as long as the tibia. 

 Ungues in every tarsus dissimilar each to the other. Mesothoracic wings large, ovate- 

 oblong, gradually rounded off from the terminal to the inner margin ; the first and 

 second axillary nervures (9^ and 9^) enclose a narrow space, which usually maintains an 

 almost even width for some distance from the inner margin and (leaving cross veinlets 

 out of account) commonly extends to the wing-roots. [In tig. 31 d, through individual 

 aberration, the functions of 9^ have evidently been usurped by an intercalar nervure, to 

 which the numeral has therefore been assigned, whilst the true first axillary nervure is 

 abbreviated and isolated.] Prtebrachial nervure of the fore wing separate from the 

 second of the following intercalar nervures, and therefore simple. iVyw^j A.— Terminal 

 margins of the mesothoracic wings free. Palpus of maxilla i. 3-jointed [probably 

 2-jointed in Callihcetis], not shorter than the lacinia ; the latter terminated at the tip 

 with either spines or strong teeth, and near the tip nude externally, but armed with 

 rigid setulte, diversely in different genera, along the adjoining portion of the inner 



SECOND SERIES. — ZOOLOGY, VOL. III. 21 



