THE JURASSIC PERIOD 



713 



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

 corals and crinoids in abundance in the later part of the period. 

 The modern (Hexacoralla) type of corals was in the ascendant (a, 6, c, 

 Fig. 484) , and formed abundant reefs, particularly in the European 



Fig. 482. The internal shell of a belemnite, restored; the lower, solid, 

 conical portion, (at the left in the Fig.) the part most frequently pre- 

 served, is the rostrum or guard; the middle portion is the phragmo- 

 cone, which is a diminutive chambered shell with septa, siphuncle, 

 and protoconch as in the older tetrab ranch order; the upper part is 

 the prostracum, which corresponds to the "pen" of the living cuttle-fish. 





Fig. 483. A GROUP OF JURASSIC PELECYPODS. 



Gryphcea arcuata Lam.; c, Ostrea deltoidea Sby.j 

 virgula D'Orb.; e, Aucella mosquensis Keys. 



a, Trigoni navis Lam.; 6, 

 d, Exogyra (Ostrea) 



