450 NORTH AMERICAN INDEX FOSSILS. 



with the first anal; anal groove absent; anterior costal cell of both 

 wing pairs darkened to a wing-spot or pterostigma; cross-veins 

 forming mostly a regular network; arcukis well developed. 



A. Triangle formed by cubitus and two cross veins from Cu to M. Anisoptera. 



B. Quadrangle formed through less abrupt bending of Cu Zygoptera. 



Fig. 1752. Odonata wings from Florissant, Colorado. «, Stenogomphus carletoni 

 Scudder, left fore wing, X f » '^> Trichocnemis aliena Scudder, right wing, X 2. (After 

 Scudder.) 



There are about 2,300 Recent and 92 Tertiary species known; 

 of the latter, 16 are American. As an example of Anisoptera may 

 be mentioned Stenogomphus carletoni Scudder (Fig. 1752,0), from 

 the Oligocene of Roan Mt., Colorado, and of the Zygoptera, Tri- 

 chocnemis aliena Scudder (Fig. 1752, &), and three species of 

 Agrion from Florissant, Colorado. Five species have been obtained 

 from the Green River beds of Wyoming (Dysagrion, three species, 

 Podagrion abortivum Scudder, and others). 



Order PLECTOPTERA Packard. 



(Ephemeridce or May flies.) 

 Delicate insects, with the mouth parts small and more or less 

 atrophied, the antennae short, the abdomen with two or three very 

 long slender cerci. Wings delicate, anterior pair always much 

 larger than posterior pair; venation variable, generally fan-like, 

 with a number of auxiliary sectors or longitudinal veins and nu- 

 merous cross-veins ; anal furrow wanting. C marginal ; Sc and R 

 always simple ; Rs originating near the base, and generally divided 

 into a number of branches ; M isolated, not much branched ; Cii 

 with one or several forks ; anal veins variable in number, often 

 repeatedly branching. 400 Recent, 18 Tertiary and i Quaternary 

 species are known ; 7 species of Ephemera occur at Florissant, 5 

 of them larvae. (Example: Ephemera howarthi Cockerell.) 



