258 HISTORICAL GEOLOGY 



Later Cretaceous Angiosperms were remarkably uniform and 

 widespread over the earth. 



Protozoans. — The Foraminijers were perhaps more prolific 

 than during any other period. Their tiny shells practically make 

 up the great chalk beds, especially those of England, France, and 

 the Gulf Coast of the United States. 



Porifers, Ccelenterates, and Echinoderms. — Among these the 

 Sponges were common; the Corals and Crinoids were not so com- 

 mon; while the Echinoids were abundant and diversified, includ- 

 ing both the regular and the irregular forms. All these forms 

 were quite modern in aspect. 



Molluscoids. — Since these organisms had previously attained 

 very much their present-day structures and subordinate position 

 among the invertebrates, they require no special mention here. 



Mollusks. — Pelecypods continued to be very abundant, with 

 the same genera of the Oyster (Ostrea) family of the two preceding 

 periods still prominent. In addition to these were many species 

 of the characteristic genera Exogyra and Inoceramus (see Fig. 

 160). Many of the other genera, often of modern aspect, were 

 also present. 



Gastropods were enriched by the appearance of many modern 

 genera. 



Cephalopods. The Nautiloids had before this become greatly 

 reduced, with only a comparatively few coiled forms of rather 

 modern aspect left. Ammonoids continued to be very promi- 

 nently represented as regards both numbers of species and indi- 

 viduals, especially by the Ammonites, of which more than 2000 

 species are known from the Mesozoic alone. Some Cretaceous 

 Ammonites attained a diameter of several feet. During the Cre- 

 taceous many of the Ammonites showed a remarkable tendency 

 to assume strange forms (Fig. 161). Some developed uncoiled 

 shells; others spiral shapes; while still others were curved or 

 actually straight (e.g. Baculites). Thus, externally at least, there 

 was a reversion to the early Paleozoic forms, but in all cases they 

 retained their complicated suture or partition structure. " These 

 strange forms have been likened by Agassiz to death-contortions 

 of the Ammonite family; and such they really seem to be. ... 

 From the point of view of evolution, it is natural to suppose that 

 under the gradually changing conditions which evidently prevailed 

 in Cretaceous times, this vigorous Mesozoic type would be com- 



