THE JURASSIC PERIOD. 



83 



The gastropods were abundant in some quarters but singularly 

 absent in others, and among them were some genera still living. 



(4) Suggestive of shallow clear seas was the reappearance of corals 

 and crinoids in great abundance in the latter part of the period. The 

 modern (Hexacoralla) type of corals had come into dominance, and 

 gave rise to reefs so abundant and so wide-spread, particularly in the 



Fig. 359. — A Group of Jurassic Pelecypods: a, Trigonia naris Lam.; b, GrypJuva 

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

 Aucella mosqucnsis Keys. 



European seas of the Middle Oolitic stage, as to give the name Corallian 

 to the epoch, and Coral Rag to the formation (Fig. 360, a and b). This 

 was a feature of the last expansive stage of the period, and seems to 

 mark the climax of base-leveled, vegetal-mantled lands, with minimum 

 inwash of silt correlated with a wide, thin sheet of epicontinental 

 water. 



The crinoids again rose to prominence, though their diversity of 

 forms was not great. They departed from Paleozoic forms in a marked 



