232 



HISTORICAL GEOLOGY 



interest because, in the embryonic development of the modern 

 Crab, the long tail of the early stage gradually becomes shorter 

 and practically absent in the adult stage. This is an excellent 

 example of the so-called "Law of Recapitulation" (see Fig. 145). 



Insects were numerous in cer- 

 tain localities at least, some hun- 

 dreds of Jurassic species being 

 known. There were many species 

 of the simpler forms, includ- 

 ing Grasshoppers. Among higher 

 forms the Beetles became very 

 abundant, while Flies and still 

 higher Insects such as Bees, Ants, 

 and Wasps made their first ap- 

 pearance in the Jurassic. Butter- 

 flies, which depend upon typical 

 flowering plants (Angiosperms), 

 were not certainly present. Proof 

 of their presence would of course 

 show the existence of the Angio- 

 sperms in the Jurassic. 



Fishes. — Selachians contin- 

 ued to be common; Dipnoans 

 were rare ; and Ganoids were still 

 the predominant Fishes. A very 

 important change, showing pro- 

 gress among the Fishes, took 

 place by the first appearance of 

 the Teleosts or true bony Fishes, 

 which types prevail today. The 

 Jurassic forms were simple, not 

 numerous, and they were fre- 

 quently on the border between true Ganoids and true Teleosts 

 (Fig. 146). 



Amphibians. — Little or nothing is known concerning Jurassic 

 Amphibians. This is in marked contrast with their prominent de- 

 velopment in several immediately preceding periods. By the close of 

 the Triassic they are known to have greatly diminished, never again 

 to rise to prominence. Only a few small forms, such as Frogs, Xewts, 

 and Salamanders, represent this once great class at the present time. 



Fig. 144 

 A Jurassic long-tailed Decapod (Ma- 

 cruran). (After Neumayr's "Erd- 

 geschichte," from Schuchert's 

 "Historical Geology," courtesy of 

 John Wiley and Sons.) 



