EVOLUTION OF FOSSIL CEPHALOPODA. 



247 



ants, and by a comparative study of larval stages with 

 adult forms the naturalist finds the key to relationships, 

 and is enabled to arrange genera in genetic series. They 

 were all marine, never parasitic, always free, and so with 

 them there is no obscuring of the record ; also in the 

 Mollusca generic and specific characters show in the 

 shell better than in the soft parts ; so the classification 

 of fossil ammonites is just as good as that of living 

 shellfish. 



The importance of this will be appreciated if we con- 

 sider other groups. The crustacean goes through suc- 

 cessive stages and moults its shell after each change, 

 thus making it difficult, especially in fossils, to find and 

 study these various stages. The ammonite, after it has 

 grown out of each stage of its shell, carries it around 

 enveloped in the later chambers. So in a single speci- 

 men the record is complete, and may be read by skilful 

 handling. 



Although genera appeared in the order of corre- 

 sponding larval stages, they did not disappear in the 

 same order; and so their survival under favourable con- 

 ditions is liable to make confusion in the record, if one 

 depends wholly on the study of series of adults. Such 

 forms, for instance, as Styrites, Tropiceltites, Miltites, and 

 others, that are now known only in the Karnic zone of 

 the Upper Trias, are undoubtedly such survivals, for 

 they still have simple goniatitic sutures, very little orna- 

 mentation, and in general are more like Lower Triassic 

 ammonites than members of the Tropites subbullatus 

 fauna. The stray Tirolites foliaceus, which appears in 

 the Alps and in California in this same fauna, is another 

 survival of a Lower Triassic type, but fortunately we 

 do know Tirolites in the horizon where it belongs. If 

 this were not the case the naturalist would be very much 

 puzzled at finding Trachyceras of the Karnic horizon 



