1 136 PRINCIPLES OF STRATIGRAPHY 



sippic (Chouteauan) has advanced into the Ceratite stage, while 

 IVaagenoceras of the Permic has already true Ammonite (phylli- 

 form) sutures. Sometimes, by retardation, a type remains in a 

 more primitive evolutional stage, one which normally characterizes a 

 lower horizon. The case of the Triassic ammonite Trachyceras, 

 cited by J. P. Smith (45), from the Karnic limestone of California, 

 and referred to in Chapter XXV, belongs here. This had persisted 

 in the more primitive Tirolites stage and so suggested correlation 

 of the beds with those of a lower horizon. The "Pseudoceratites" 

 of the Cretacic (Hyatt-23) form another instructive example. In 

 these types (Protengonoceras, Engonoceras,* etc.) arrestation in 

 development affects the later stages (the earlier stages being ac- 

 celerated with reference to the corresponding stages of their Juras- 

 sic ancestors), so that the adult sutures remain in the ceratite or 

 even goniatite stage. This is a case of heterepistasis, the cessation 

 in development affecting only the sutures. What appears to be a 

 good example of corresponding stages in development in distinct 

 provinces at about the same time period, thus serving for inter- 

 regional correlation, is seen in the case of Clavilithes of the Parisian 

 Eocenic, and the corresponding morphologic equivalents of the 

 Eocenic of the Gulf States of North America. The American 

 series of species parallels the Parisian to such an extent that they 

 have been regarded as varieties of the Parisian species. There is 

 every reason for believing, however, that they represent an in- 

 dependent development. (Grabau-15.) 



The graptolites present other examples of corresponding stages 

 in development reached in distinct genetic series at approximately 

 the same time. Thus, Dichograptus, Tetragraptus, Didymograptus, 

 etc., represent stages in development, rather tlian monophyletic 

 genera, but, since these stages appear simultaneously in the various 

 lines of descent, they may be used as geologic genera, eminently 

 adapted for correlative purposes. (Ruedemann-38.) 



(d) Correlation by faunas and floras. Representative species. 

 When the index species themselves are not represented, correlation 

 by means of the sum total of associated organisms must be made. 

 Thus Paradoxides is absent from the IMiddle Cambric of the Ap- 

 palachian and Pacific provinces of North America, nor are any 

 of the associated species of the other genera present in these faunas. 

 Representative species, however, occur and the sum total of the 

 IMiddle Cambric faunas of the various provinces has similarity 

 of expression, which is almost as good as absolute identity. The 

 lower Middle Cambric of the Atlantic Coast and of western Europe 

 * For illustrations see North American Index Fossils, Vol. II. 



