14 SCUDDER ON THE DEVONIAN 



then be an elevated, and the mediasthial a depressed vein (see fig. 8a), which is never the 

 case in such insects. The nervuie at the margin then is certainly the marginal, and 

 that next to it the mediastinal vein. Only one vein, the scapular, can lie between the med- 

 iastinal and the externomedian, yet between our unloubted mediastinal and our presumed 

 externomedian there are no less than three veins to be disposed of. 



Two of these lie in the depression following the mediastinal vein, while the third is 

 upon the side or the upper edge of the ascending portion of the area, which on the 

 opposite side of the depression lies at the level or above the level of the mediastinal vein (see 

 fig. 8a). It seems, therefore, highly probable that the two low-lying veins are branches 

 of a scapular vein which probably divides not much further toward the base ; and that 

 the third vein in question is the main externomedian stem, of which the branching 

 vein below is only a principal basal offshoot ; indeed the very fact that the branches of 

 this ofishoot are thrown ofT from its superior surface leads to the presumption that it is 

 itself a branch from a vein above ; for, while an area between two branches of one vein 

 may not very infrequently be filled by superior offshoots from an inferior branch, it would 

 certainly be abnormal for a wide area to be filled by superior offshoots from an upper 

 branch, or even from a main stem itself Presuming then upon the correctness of these 

 interpretations, the structural basis of the wing is as follows : 



The marginal vein forms the border. The mediastinal vein is simple, and, 

 running nearly parallel to the marginal vein, probably terminates by impinging 

 upon it not very far from the middle of the outer half of the wing; froni it run 

 frequent oblique delicate cross veins to the border. The scapular vein divides into 

 two longitudinal veins before the middle of the wing, prolxibly considerably before 

 it ; for even before the middle of the wing, and for as great a distance beyond 

 it as it can be traced, the two branches are exactly parallel to each other and 

 the mediastinal ; all the longitudinal interspaces in the middle of this part of the 

 wing are equal ; the forks are connected with each other (and the upper with the medi- 

 astinal?) by tolerably frequent faint cross veins at right angles to the nervures ; and in 

 the middle of the wing and beyond it, at least for a short distance, have a gentle upward 

 direction, and even curve very slightly, almost impeixeptibly, in the same direction ; 

 beyond however, they must curve strongly in the opposite direction, for the pair of detached 

 veins toward the tip of the wing have a decided downward direction, and these forks, 

 whether the same or not, must in that part of the wing have a similar direction ; probably 

 they are the same, and if" so they show that tliey retain a similar distance apart until 

 they strike the costal margin, one just before or at the tip, the other a little earlier. 



The externomedian vein must divide into two principal veins near the base of the wing ; 

 the upper branch follows closely the course of the veins above, and lies as far from the near- 

 est as the latter from the next ; a little beyond the middle of the wing, however, this space 

 is slightly increased, and an intercalary vein, straight and similar to the others, but fauiter, 

 takes its rise from an oblique bent cross vein ; all the other cross veins in this interspace 

 and on either side of the intercalary vein, are like the others in the scapular interspaces, 

 and the whole area in which these straight and directly transverse cross veins lie, namely 

 that between the mediastinal and upper externomedian veins, forms a deeply sunken but 

 broad sulcus, the floor of which is nearly flat, and not V-shaped as usual in folds in this 



