§ 46. CRUSTACEANS OF THE CARBONIFEROUS SYSTEM. 729 



It is in the carboniferous system that we first meet, in a 

 descending order, with vestiges of the extinct family of 

 crustaceans called Trilobites (Medals, p. 552) : but as 

 these animals are especially characteristic of the older rocks 

 — the Silurian — I shall reserve a particular notice of them 

 for the next discourse. 



Insects. — The remains of insects belonging to several 

 genera have been found ; from the ironstone nodules of 

 Coalbrook Dale, several species of beetles, related to the 

 Cui'culio, or diamond beetle, have been obtained. In a 

 nodule from the same locality I discovered the wing of a 

 large neuropterous insect, closely resembling a species of 

 living Corydalis of Carolina.* 



iron-stone, are thus explained by Sir H. De la Beche : — " The argil- 

 laceous iron-stones are formed of carbonate of iron, mingled mecha- 

 nically with earthy matter, commonly corresponding with that 

 constituting the shales with which they are associated. Mr. Hunt, 

 of the Museum of Economic Geology, instituted a series of experiments 

 to illustrate the production of these clay ironstones, and he found 

 that decomposing vegetable matter prevented the further oxidation of 

 the protosalts of iron, and converted the peroxide into protoxide of 

 iron, by taking a portion of its oxygen to form carbonic acid. Under 

 the conditions necessary for the production of the coal distributed 

 among the sand, silt, and mud, the decomposition of the vegetable 

 matter would necessarily form carbonic acid, among other products. 

 This carbonic acid, mixed with water, would spread with it over areas 

 of different dimensions according to circumstances ; forming salts and 

 meeting with the protoxide of iron in solution, it would unite with 

 the protoxide, and form a carbonate of iron. The carbonate of iron 

 in solution would mingle with any fine detritus which might be held 

 in mechanical suspension in the same water ; and hence, when the 

 conditions for its deposit arose, which would happen when the need- 

 ful excess of carbonic acid was removed, the carbonate of iron would 

 be thrown down intermingled with the mud ; and if not in sufficient 

 quantity to form continuous layers, would aggregate into nodules, and 

 be arranged in planes amid the sediment." — Memoirs of the Geological 

 Survey of Great Britain, p. 185. 



* Medals of Creation, p. 578. This specimen is now in the British 

 Museum. 



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