NO. 2146. SOME AMERICAN FOSSIL INSECTS— COCKERELL. 101 



Sellards. The distinction between the Spanioderidae, Geraridae,' 

 and Ischnoneuridae becomes more difficult with the discoA-ery of 

 additional types. The Spanioderidae have few branches to the 

 radial sector, and many to the cubitus; in the Geraridae the radial 

 sector has many branches, and the cubitus is more or less reduced. 

 Ischnoneura has four branches to the radial sector, and four (five?) 

 to the cubitus; it thus falls between the other two families. The 

 general structure of the body and legs seems to be about the same in 

 all. Danielsiella may be referred to the Geraridae, but perhaps the 

 correct solution would be to recognize a single family Geraridae 

 (Gerarina Scudder, Geraridae Handlirsch), with subfamilies 

 Gerarinae, Spanioderinae (Spanioderidae Handlirsch) and Ischno- 

 neurinae (Ischnoneuridae Handlirsch). 



It may be convenient to adopt a formula for the venation. The 

 number of primary branches may be expressed by a figure, and if 

 these again branch a second figure may be added, and a third for 

 tertiary branchlets. According to this method the formula for the 

 radial sector, media and cubitus of S^aniodera ambulans Hand- 

 lirsch will be : 



Anterior wing. Es. 3. 1. M. 3. 1. Cu. 5. 



Hind wing. Es. 2. M. 1. Cu. 5. 



If necessary, distinction can be made between upper and lower 

 branches, by using the form of a fraction ; thus Danielsiella has Es. 

 f , M. 3. It is not always easy to distinguish the main stem from an 

 upper branch (though in Danielsiella there is no doubt), and the 

 anterior wing of Spaniodera ambulans should perhaps be interpreted 

 as having Es. 2. 1. 1. 



ODONATA. 



LITHRAGION (?) OPTIMUM, new species. 



Length, about 51 mm., the thorax about 9 mm. ; thorax and abdo- 

 men with black markings; mesepisternal plates with a broad black 

 band covering most of the surface and a narrow band along inferior 

 margin, with a narrow pale stripe, curved mesad posteriorly, be- 

 tween the two dark portions; mesepimeron with a broad black band, 

 invaded below near the anterior end by an oblique pale stripe; ab- 

 dominal seg-ments in lateral view showing a pair of stripes, one 

 dorsal and one subdorsal, failing anteriorly on the middle segments, 

 but the subdorsal stripes shorter, with the anterior end pointed; 

 ventral side of abdomen dusky; legs slender, hind femora about 7 

 mm. long; tibiae very slender, with the usual bristles, hind tibiae 

 6 mm. long; wings 36 mm. long and about 7.5 wide, duslvy, the apical 

 margin suffusedly darker; nodus 11.5 mm. from base of wing; two 

 antenodal cross- veins, in the usual position; quadrilateral with 



