200 EEV. A. E. EATON ON EECENT EPHEMERID^ OE MAYELIES. 



of BiBtisca are concealed by a larg-e notal shield when the insect becomes adolescent, 

 and their structure is j)eculiar. 



Section 10 of the Genera. — Type of SipJdurns. Adult. — In the fore wing, the first 

 axillar nervure (9') meets the inner margin at or before the middle ; the anal nervure 

 (8) terminates close to the anal angle and receives sevei'al simple or divided branchlets 

 [intercalaries] from the outer half of the inner margin. Pronotum well developed. 

 Nymph. — Palpus of maxilla i. as long as the lacinia, or longer. Pronotum and sutures 

 of the mesonotum well defined. Terminal maro:ins of the fore win2:s free. 



The affinity between the genera of this section and those of the type of Baetis has 

 already been remarked upon at page 15i. i?<g^/«c«, ranked apart in Section 11, is rather 

 similar to genera of the Sqjhliirus type in aspect. 



Tlie relative dimensions of legs, sette, or segments, indicated in the definitions of 

 the genera, are employed, in the absence of whole figures, to convey an approximately 

 definite idea of the aspect of typical examples of the insects concerned, so far as this 

 may be dependent largely upon those proportions. But yet the descriptions must not 

 be interpreted rigidly in these particulars ; because the geuerical likeness of species can 

 l)e maintained and be appreciated at a glance, where the standard proportions have been 

 departed from in some points. 



Nymphs of only three of the genera of- this section are known ; that of Chirotonetes 

 (as it is supposed) has tracheal branchiae on segments 1-7 of the abdomen, all of which 

 are single obovate lamellte ; SipIUio'us has similarly situated tracheal branchiae, but those 

 of segments 1 and 2 are double. Onlscigaster has tracheal hranchiye on segments 1-6 

 only, of which all but the foremost are produced each on the inner side into a curiously 

 foliated lobe. 



Tabulation of the Genera of the Siphlurus Tvpe. 



Tibiae of the liiuder legs longer than, or at least subequal in length to, the tarsi. Prox- 

 imal joint of the hind tarsus 

 shorter than the 2nd joint. Tarsal claws mutually 



dissimilar. Costal dilatation of the hind wing acute. In ? abdomen the 

 ventral lobe of segment 9 is bifid and acutely excised ; pleurae 

 of segments 7 and 8 squarely truncate, each with a minute 

 spinule at the point, those of segment 9 produced each into 

 a short acute triangular tooth. Diminishing gradation of 

 tarsal joints : — fore tarsus ,$ 2, 3, 4, 5, 1, ? 5, 2 sub- 

 equal to 3, 4, 1 ; hind tarsus ^ and ? 5, 2, 1 subequal to 



3, 4 [in ? PL XVIII. 32 b, 5, 2 subequal to 3, 1, 4] Coloburus. 



alike, narrow and liooked. Costal dilatation of the hind wing obtuse. In 

 $ abdomen, the ventral lobe of segment 9 is 

 roundly emarginate with acute points ; pleurae of segments 7 and 8 

 squarely truncate, of segment 9 narrowly dilated outwards 

 and produced each into a short acute tooth behind. Grada- 

 tion of tarsal joints: — fore tarsus (J 1, 2, 3, 4, 5, ? 5, 1, 

 2, 3, 4 ; hind tarsus (5' 5, 2, 1 subequal to 3, 4, ? 5,1 



subequal to 2, 3, 4 Chirotonetes. 



acutely excised, with short acute points; pleuraj of segments 7-9 



