154 BRITISH APHIDES. 



The alternation of marine and fresh-water forma- 

 tions frequently to be observed in these places is 

 generally ascribed by geologists to corresponding 

 upheavals and depressions of the existing sea-bottoms. 

 Thus irruptions of the sea seem to have been frequent, 

 and caused the intermingling of land and marine 

 remains. Seals, sea-mollusks, and sharks may be 

 from such causes closely located with trunks of trees, 

 lizards, Aphides, and leaves of land plants. 



In the Lower Miocene beds insects are numerous, 

 but examples of Hemiptera at present discovered are 

 comparatively few. Though Aphides have not yet 

 shown themselves amongst these sediments, the Aphis- 

 feeding Syrphidae and Ooccinellidaa would indicate that 

 they were contemporaneous with other Rhynchota. 



The marls of Radoboj, in Croatia, which belong to 

 the Middle Miocene, possess much interest; for they 

 contain many fine impressions of Aphides ; and as we 

 ascend to the Upper Miocene, specimens become much 

 more plentiful ; and thus we come to the famous insect 

 beds of CEningen. These calcareous deposits formed 

 the floor of a considerable fresh- water lake tying in 

 the Rhine Valley between Constance and Schaffhausen. 

 The sediment has been very favorable to the preser- 

 vation of insects, the casts from their wings being in 

 many cases very perfect. Even midges, with the 

 hairs on their antennaa and legs, are clearly recog- 

 nisable under the microscope. This will show how 

 swiftly the insects must have been submerged and 

 covered by mud before there was time for their 

 decomposition. 



This mass of insect remains occurs in an exceedingly 

 fine-grained limestone, scarcely exceeding an inch in 

 thickness. The stratum is so laminated that it may 

 be cleaved into about 250 paper-like slabs, of such 

 delicacy that even the original colours of the insects 

 may sometimes be seen, as if they had been painted 

 on the matrix. 



Upwards of 5700 specimens have been examined 



