26 



A vegetable stem, very strongly marked, lies right across the wing 

 just distally from the nodus. The stem is actually beneath the wing, 

 since the main veins can be followed easily across it. 



Under strong oblique light, it is just possible to see two nearly 

 parallel oblique marks passing from M2 to Rs in the region just 

 distally below the curve of M2 (left clear in the figure). One of 

 these is definitely determined to be a light .scratch ; the other might 

 possibly be the partly obliterated remains of the oblique vein. Much 

 as one would like to determine the oblique vein definite^ and thus 

 know whether the specimen should be placed in the Anisoptera or 

 in the Epiophlebiinse (and it should be remarked that the oblique 

 vein ought to occur just at this point), yet it cannot be said that this 

 important vein has been satisfactorily made out, and so it is preferred 

 to leave it undetermined. The same may be said of. the origin of 

 the vein provisionally labelled Rs ; the wing is just hereabouts in 

 very poor condition, and none of the veins can be closely followed 

 out. On the other hand, the perfect preservation of the nodus and 

 pterostigma is indeed marvellous. Even the slight ' ribbing " of 

 the costa just distad from the nodus can be identified by careful 

 manipulation of the light, while the pterostigma is still more easily 

 picked out as a somewhat darkened and thickened patch near the 

 wing-tip, enclosed by veins whose outline is absolutely perfect. The 

 alternate convexity and concavity of the main veins are also easily 

 distinguished. 



MEASUREMENTS : Greatest length of fragment 21.5 mm., 

 greatest breadth 11 mm., distance from nodus to pterostigma. 12 

 mm., length of pterostigma 2.8 mm. 



TYPE: Spec. 3a (Plate 4, fig. 2). TYPE-COUNTERPART: 

 .Spec. 3b. (B. D. Coll.) 



THE PHYLOGENY or THE NODUS IN THE DRAGON-FLY WING. 



The discovery of Mesophlebia gives \is a very important clue 

 to the phylogeny of that extraordinary and unique formation in the 

 Dragon-fly wing known as the nodus. So far its origin has been 

 only speculated upon. Now at last we have some solid ground on 

 which to base its phylogeny. In Mesophlebia the nodus is very 

 beautifully preserved, and it is also of a form not found in airv recent 

 Dragon-fly wing. It is shown enlarged in text-fig. 4. It will be 

 seen that the nodus is made up of two distinct parts, one distal from 

 and the other proximal to the nodal joint. The proximal part is 

 formed by the distal end of Sc being turned up and fused with the 

 costa ; this fusion is thickened and beautifully rounded off. The 

 distal part is obviously formed simply by a specialised cross-vein, 



