330 



HISTORICAL GEOLOGY. 



period. We shall touch very briefly all except the most 

 important characteristic kinds. 



FIG. 263. Zarnia spiralis, a living 

 cycad of Australia. 



FIG. 264. Stem of cycadeoidea 

 megalophylla. 



Crinoids, beautiful, plumose-armed, and lilylike, are 

 abundant (Fig. 267) ; but so, also, are the free asteroids 

 and echinoids (Fig. 268). The two kinds, stemmed and 

 free, are evenly balanced. 



Bivalves are, of course, abundant and of characteristic 

 forms, in this as in all geologi- 

 cal times ; but we can only 

 draw special attention to the 

 oyster family (including Os- 

 trea, Grypliea, Trigonia, etc.), 

 which were first introduced 

 here (Figs. 269-271). 



Ammonites. The Ammo- 

 nite family were introduced first 

 in the Devonian as Goniatites. 

 These were replaced in the Tri- 

 assic by .Ceratites. The Am- 

 monites proper, the highest 

 type of the family, were intro- 

 duced in the early Mesozoic, culminated here in the Juras- 

 sic, continued through the Cretaceous, and died out at its 



FIG. 265. Jurassic plants: Ptero- 

 phyllum comptum (a cycad). 



