EEV. A. E. EATOX OX EECEXT EPHEMEEID.E OR MAYFLIES. 21 



alterations have been made (such as the establishment of genera in place of provisional 

 sections) and a remarshalling of the members of certain alUances ; and besides this, some 

 genera, then isolated, have become rallying-points of new alliances ; but these changes 

 have not disturbed the scheme as a whole. The plan upon which it has been drawn up 

 may be described as based upon conclusions derived from comparisons of adult insects 

 checked and modified by others educed from the study of younger specimens. The o-eneral 

 effect of this plan has been to bring into the middle of the series genera of hardy habit 

 that moult completely at the last ecdysis, that have functional legs with four distinct 

 tarsal joints, and have the oculi of the adult male either bipartite or ascalaphoid. The 

 ends of the series are occupied by genera that have the oculi of the d furrowless and 

 undivided, those having weak or functionalless hinder legs with at most four distinct 

 joints to the tarsus, whose eggs are discharged in bulk, and whose Life is truly ephemeral, 

 commencing the series ; and those whose legs are all efficient with five distinct joints to 

 the tarsus, whose oviposition is gradual, whose life in the adult condition is measured bv 

 days, if it attain its full natural term, and whose last moult is complete, bringing the 

 series to its close. The scheme is open to objections attendant upon all linear arrano-e- 

 ments in zoology, individual genera here and there having to be ranked in groups with 

 whose formulated definitions they are largely at variance during some portion of their 

 existence. The subjoined Table exhibits the system of classification ; generical details of 

 the adult flies are illustrated in the first 24 plates, and the numbering of the genera 

 quoted in the Table accords with the numerals assigned to them in the writing of those 

 plates. 



Table of the Classificatiox of Genera of the Ephehered^ 



NUMBERED AS IN PlATES I.-XXIV. 



Family EPHEMERID^. 



Group I. 

 Series I. 



Section 1 of Palingenia Genera 1-6. 



„ 2 oi Polymitarcys „ 7-10. 



Series II. 



Section 3 of Ephemera Genera 11-13. 



Group II. 

 Series I. 



Section 4 of Potamanthus Genera IJ^IS. 



Series II. 



Section 5 oi Leptophlebia... Genera 16-2S. 



,, 6 oi Ephemerella „ 24, 24- dis &c. 



Series III. 



Section 7 oi Ctenis Genera 25, 25 ii«, 26. 



„ 8 of Prosopistoma 



27. 



Serie IV. 



Section Q oi Baetis Genera 28-31. 



