THE JURASSIC PERIOD 



Fig. 430. GROUP OF JURASSIC PELECYPODS. a, Trigonia navis Lam.; b, Gry- 

 phcea arcuata Lam.; c, Ostrca dcltoidea Sby.; d, Exogyra (Ostrea) virgula D'Orb.; e, 

 Aucella mosquensis Keys. 



(6) The trilobites of the sea, and the eurypterids of land waters, 

 had been succeeded by decapods which rose to a moderate and pro- 

 longed ascendancy. The prawns and lobsters (Macrura, long- 

 tailed decapods) were the earlier division, and the most numerous in 

 this period; but the first known crabs (Brachyura, short-tailed 

 decapods) appeared before the period was past. The macrurans 

 seem to have frequented embayments and protected locations near 

 the land, or perhaps within it, for terrestrial, fresh- water, and marine 

 species are preserved in the same sediments. Probably macrurans 

 had representatives in terrestrial waters then, as now. 



(7) Sponges and foraminifera abounded and are well preserved. 



(8) The marked change in the aspect of the fishes which set in 

 during the Trias was carried farther in this period. Some of the 

 older types declined; but the selachians (sharks) remained abundant, 



