532 



HISTORICAL GEOLOGY 



Crustaceans. — Crustaceans (Fig. 496 A, B) of a very modern 

 appearance took the place of the trilobites and eurypterids. In 

 America few fossils of this group have been found, but in the Jurassic 

 lithographic limestone of Bavaria many beautifully preserved speci- 

 mens have been col- 

 lected. It is possible 

 that this class was as 

 abundant in America 

 as in Europe, but if 

 so, the conditions for 

 the preservation of 

 the remains were not favorable. 

 Ancestral long-tailed crustaceans 

 of the lobster and shrimp type 

 (Macrura) began in small num- 

 bers in the Triassic, and a few 

 survivors of these ancient forms 

 are living in the deep seas of the 

 present. Crabs are, in general, 

 crustaceans of the lobster type 

 in which the tail is abbreviated 

 and turned under the body and 

 the shell widened and otherwise 

 modified (Brachyura). Crabs 

 did not appear until the Jurassic 

 and were derived from the long- 

 tailed series, as numerous speci- 

 mens intermediate between the 

 two show. 



Insects. — Insects are better 

 known from the Jurassic than 

 from any other portion of the 

 era, probably, as in the case of 

 the crustaceans, because the conditions favorable for the preservation 

 of their remains were better than at other times. True cockroaches 

 and beetles are known from the Triassic; and practically all of the 

 groups of to-day were present in the Jurassic, with the exception 

 of those depending upon flowering plants for their food. Crickets, 

 locusts, and cockroaches (Orthoptera), May flies, dragon flies, and 

 caddis flies CNeuroptera) occur. Wood beetles are found associated 



Fig. 496. — Mesozoic crustaceans : A, 

 Penceus meyeri (Jurassic) ; B, Eryon pro- 

 pinquus (Jurassic). 



