MESOZOIC ERA 



175 



was the complexity of fluting of the margins of the septa. The intricacy 

 of these Hnes suggests the tracery of frost patterns on a windowpane, or 

 the outlines of a fern frond (Fig. 9.1). 



The Jurassic period (Table 7.1, p. 137) saw the culmination of the am- 

 monites. They continued into the Cretaceous in diminished numbers. Dur- 

 ing the later stages of their evolution bizarre shell forms occurred. Some 

 shells showed a partial or complete tendency not to coil. Depending upon 

 the degree of this tendency, loosely coiled, bent, or straight shells resulted. 



FIG. 9.2. Belemnite, restored; length 5 or 6 feet. (Mainly after Hussey, His- 

 iorical Geology, McGraw-Hill Book Company, Inc., 1947.) 



Some shells showed coiling of the first portion to be formed, followed by 

 subsequent formation of a straight shell section. The tendency of any 

 group of animals to develop such an assemblage of bizarre and atypical 

 forms is sometimes called racial "old age" or "senescence" (p. 153). Term- 

 ing it so gives no adequate explanation for its occurrence, however. Yet 

 such diverse animals as trilobites, ammonites, and dinosaurs exhibited the 

 tendency. No ammonites survived the close of the Mesozoic. 



We should note that the more conservative group of cephalopods, the 

 nautiloids, did not share the extinction of their relatives the ammonites. 

 We recall that the nautiloids were the first cephalopods to appear (p. 



