XVI] THE GEOLOGICAL RECORD 317 



with carbonate of lime. The region was volcanic ; hot springs and 

 geysers probably occurred. Thus we have at once a comparison 

 and a contrast with Florissant. For, on the one hand, the general 

 conditions were somewhat similar; but, on the other hand, the 

 altitude was very much less. In a low-level lake at the present 

 day, we should expect to find an Odonate fauna chiefly consisting 

 of Libellulinae and Agrionidae, with some Aeschninae, while a few 

 Calopterygidae might visit the lake, though breeding in neighbouring 

 rivers. This is just the kind of fauna revealed to us by the 

 Oeningen beds. 



A very extraordinary circumstance is the occurrence at 

 Oeningen of a very large number of Libellulid larvae, while the 

 imagines are much less common. In a thin stratum from one 

 of the quarries, larvae in all stages of growth were found jumbled 

 together in perfect swarms. Some had the labial mask projecting 

 as if in the act of striking their prey. The only possible explanation 

 of this phenomenon seems to be that, in the spring of one year, 

 before many of the imagines were on the wing, a sudden catastrophe 

 overtook the lake-bed. Through volcanic agencies, either by an 

 eruption, or a sudden uprush of hot water, or a burst of poisonous 

 gases, these larvae met a sudden and violent end, and died in 

 their natural positions 1 . 



The Oeningen specimens are not well preserved, the rock in 

 which they are enclosed being rather hard and brittle, and cleaving 

 very unevenly. In such a case, specific and even generic deter- 

 minations are not to be absolutely relied upon. The fauna seems, 

 however, to have been undoubtedly closely allied to that of 

 Holarctica as we know it at the present day. 



General Conclusions. 



The Cockroaches are the only group of insects in which the 

 geological record is so complete that a fairly accurate phylogenetic 

 tree can be constructed. The record of the Odonata, as insect 

 records go, must be reckoned a rich and valuable one. Moreover, 

 there are hopes that more careful working of some of the lesser 

 known beds, and perhaps also the discovery of new ones, may 

 yet fill up the wide and lamentable gaps to which we have at 



1 Die Urwelt der Schweiz, 0. Heer; Zurich, 1879. 



