MESOZOIC ERA: THE AGE OF REPTILES 533 



with driftwood in the Jurassic; and flies (Diptera), plant lice, and 

 aquatic bugs (Hemiptera) are known. The absence of insects depend- 

 ing upon the pollen and nectar of flowers is probably indirect evidence 

 that flowering plants were not yet in existence in the Jurassic. 



REFERENCES FOR INVERTEBRATES 

 Chamberlin and Salisbury, — Geology, Vol. 3, pp. 48-58 ; 80-94 5 134—136 ; 186-190. 

 Grabau and Shimer, — North American Index Fossils. 



Scott, W. B., — An Introduction to Geology, pp. 670-673; 684-690; 716-718. 

 Zittel-Eastman, — Textbook of Paleontology, 2d ed. (for description and bibliography). 



Fishes and Amphibians 



At the beginning of the Mesozoic, less modification in structure 

 is noticeable in this class than in others to be considered, but the 

 changes were by no means inconsiderable. 



The shark tribe (Fig. 497 A) has had a long and varied history. 

 It began in the Silurian and abounds still in the warm seas of the 

 present. These fish were abundant, both in species and individuals, 

 in the Lower Carboniferous (Mississippian), but during the Permian 

 declined rapidly, almost to extinction. In the early Mesozoic, 

 however, they once more began to increase and were common before 

 its close. The cobblestone-pavement toothed shark lived on and is 

 represented to-day by one genus, the Port Jackson shark (Cestracion). 

 A possible explanation of this curious fluctuation is as follows : 

 in the Carboniferous, being the most powerful animals and having no 

 enemies, they multiplied until their increase was checked by their 

 very numbers. Then the overspecialized forms and those that 

 failed to respond to changed conditions dropped out, leaving the 

 best to survive. These, then, gradually increased to the Middle 

 Tertiary when, through a change in climate, they became again 

 comparatively rare. (Lucas.) 



The skates and rays (Fig. 498) are sharks in which the body has 

 been admirably adapted to bottom living and probably should be 

 regarded as " the culminating forms of the specializing, bottom- 

 living sharks of the Mesozoic." (Dean.) The skates of the Meso- 

 zoic and Tertiary, without doubt, mimicked the color of the ocean 

 bottom and glided along inconspicuously, just as their living descend- 

 ants do. The teeth of all skates are simple, crushing, pavement 

 teeth, suited for crushing the shellfish and crustaceans upon which 

 they live. They are first known in the Jurassic and are abundant in 

 the seas of to-day. 



