TRICORYTHUS, A GENUS OF MAYFLIES I37 



with the nameless one from Colorado, figured by Eaton on his Plate 

 39. It agrees with that also in the robust form, but differs in the struc- 

 ture of the head, etc. 



If the Boulder nymph really belongs to Tricorythus, as seems hardly 

 doubtful, it apparently follows that that genus has been derived from 

 the Ephetnerella series by the loss of the hind wings and reduction 

 of the venation, and has really no intimate relationship with Ccenis. 



