312 



THE GEOLOGICAL RECORD 



[CH. 



preserved is Aeschnidium densum (fig. 163), remarkable for the 

 width of the wings, and the immense number of cross-veins and 

 tiny areolets filling up the spaces between the main veins and their 

 branches. Between C and Sc prenodally, and again between C 

 and R postnodally, these areolets form two or more rows. The 

 boundary between each pair of rows tends to become a straight 

 line, thus forming a slender longitudinal vein parallel to the main 

 veins. By this means there arose an apparent prolongation of Sc 

 beyond the nodus, comparable with a similar formation in the recent 

 genera Telephlebia (fig. 19 D) and Staurophlebia, but more extensive. 

 The Aeschnidiinae seem to have died out in Cretaceous times ; 

 unless,, indeed, the recent Chlorogomphus (fig. 119) be a close ally 

 or a descendant of them. 



Fig. 163. Wings of Aeschnidium densum Hagen ( x 1-3), Upper Jurassic, 

 Solenhofen. From Handlirsch, alter Deichmuller. 



There are two outstanding features of the Solenhofen beds. 

 These are, firstly, the extreme fineness of the grain of the stone, 

 and, secondly, the great scarcity of fossils. With regard to the 

 first, the Solenhofen slate was undoubtedly formed from a very 

 fine sediment deposited in shallow marine bays and inlets, and 

 derived from the adjacent land. In such places, Crustacea, 

 Fishes and Reptiles lived together, and their remains are beautifully 

 preserved for us in the fine-grained slates. Insects do not strictly 

 represent a part of the true fauna of these bays. But a number 

 of insects., including Dragonflies, were carried thither, probably 

 by floods, from the neighbouring streams. Hence their remains 

 are mingled with those of the true fauna. With regard to the 

 second feature, the paucity of fossils it must be clearly grasped 



