METHODS OF CORRELATION 1135 



divisions. Thus Olciicllus and Holinia characterize the Lower Cam- 

 bric, Paradoxidcs the ]\liddle Cambric, and Olcnus and Dikclloccph- 

 alus the Copper Cambric* Again, the Middle Cambric may be 

 subdivided into a number of zones, each characterized by a species 

 of Paradoxides. These species are either identical or representative 

 in the corresponding zones of the East American and West Euro- 

 pean Middle Cambric. 



The dendroid graptolite Dictyoncma may serve as an example 

 of a genus of more extended range, some of whose species are, 

 nevertheless, good index fossils. The genus itself begins in the 

 transition beds from the Copper Cambric to the Lower Ordovicic, 

 where D. iJabclIifoniic is a characteristic index fossil and is 

 of almost w^orldwide distribution. Other species characterize the 

 Siluric and still others the Devonic. 



(c) Correlation by equivalent stages in development. Among 

 organisms characterized by community of descent corresponding 

 stages in development are sometimes reached in different lines of 

 evolution at about the same time period. Such homceomorphic 

 representatives (morphological equivalents, see Chapter XX\') 

 may thus serve as indices of a given horizon even where inter- 

 communication has not occurred. Thus Goniatites the world over 

 characterize the upper Palaeozoic, but Goniatites are derived along 

 different lines of descent. f The simpler types along the various 

 lines of descent characterize the Devonic, while those with more 

 complicated sutures,! greater involution, or marked ornamentation 

 are mostly characteristic of the Mississippic and Carbonic. The 

 Ceratite type, in which the lobes of the sutures are further modi- 

 fied by secondary indentations, while the saddles are entire, are 

 typically developed along the various lines of evolution, in the 

 Trias. Finally, the Ammonite type, in which both lobes and saddles 

 are modified by additional indentations, appears chiefly in the Juras- 

 sic, in the various evolutional lines, and continues into the Cretacic. 

 Owing to these facts the orginal idea prevailed that Goniatites con- 

 stituted one genus, characteristic of the formations from the Devonic 

 to the Carbonic ; Ceratites formed another genus characteristic of 

 the Trias ; while Ammonites was regarded as a genus characterizing 

 the time from the Jurassic to the Cretacic inclusive. 



It is now known, however, that many exceptions to this general 

 rule exist. Genetic series,' in which acceleration of development 

 prevailed, reached the Ceratite or even the Ammonite stage in pre- 

 Triassic time. Thus the genus Prodo mites from the Lower Missis- 



* For illustrations see Grabau and Shinier, North American Index Fossils 

 Vol. II. t See Chapter XXV, p. 978. % See Chapter XXIV, p. 945. 



