230 



HISTORICAL GEOLOGY 



Fig. 140 

 An irregular or bilater- 

 ally symmetrica] Echi- 

 noid, Hemiaster tex- 

 anus, of Cretaceous 

 age. (After Hill and 

 Vaughn, U. S. Geolog- 

 ical Survey, Folio 76.) 



were reduced to only a few. Most of these 

 genera have continued to the present day, 

 so that in the succeeding periods the evolu- 

 tion of these creatures, so very prominent 

 in all of the earlier f ossiliferous periods, has 

 but little interest. 



Mollusks. — Pelecypods were even more 

 abundant than in any preceding period, 

 their shells often largely constituting whole 

 strata or thick beds. They were quite mod- 

 ern in appearance, many genera being those 

 which still exist. The members of the Oyster 

 family were most common, being represented 

 by such genera as Ostrea, Exogyra, Gryphea. 

 Gastropods, including various existing 

 genera, were usually common. 

 Among Cephalopods the ancient and important straight-shelled 

 Orthoceras had disappeared with 

 the preceding period, but coiled 

 Nautiloids still were common. 

 The Ammonoids reached the 

 very height of their develop- 

 ment in the Jurassic. Among 

 these, the most characteristic 

 and abundant were the Ammo- 

 nites, in which the sutures or 

 partition structures reached the 

 highest degree of complexity 

 (see Fig. 141). Many hundreds 

 of species are known, and often 

 Jurassic strata are chiefly com- 

 posed of them. Of all coiled 

 Cephalopods, the largest were 

 of this age, some Ammonites 

 having attained a diameter of 

 several feet. In many cases 

 "erratic and degenerate de- 

 velopments showed themselves 

 by uncoiling and strange coil- 

 ing, presaging a stage of 'sport- 



Fig. 141 

 An Ammonite with part of shell re- 

 moved to show the very compli- 

 cated (frilled) sutures. (From Nor- 

 ton's "Elements of Geology," by 

 permission of Ginn and Company, 

 Publishers.) 



