978 PRINCIPLES OF STRATIGRAPHY 



Examples of such equivalents could be multiplied indefinitely. In 

 the majority of cases, where classification has taken account only 

 of the adult characters, such homoeogenetic types have been classed 

 together as of the same genus or species, and sometimes even as 

 varieties of the same species. 



It is hardly going too far to say that the majority of classifica- 

 tions of invertebrates now in vogue may be likened to a more or 

 less warped plane passed through the crown of a tree ; all the 

 points of intersection of the branches and the plane are considered 

 as species, while all the neighboring and some distant ones are 

 grouped together in a genus, or family. The illogical nature 

 of this method, which is the natural result of a consideration of 

 adult characters only, must be apparent. 



Morphologic equivalents have been recognized most frequently 

 among ammonoids. Formerly all types in which the suture was 

 composed of simple lobes and saddles were classed as Goniatitcs. 

 All those in which the lobes were crenulated as Ceratitcs, and all 

 those in which both lobes and saddles had become compound were 

 placed in the genus Ammonites. It is now known that many and 

 diverse genetic series started with genera of the goniatite type, 

 branched out in diverse genera with ceratite sutures and that from 

 these latter were derived the genera in which the suture was am- 

 monitic. Again, similar types of ornamentation and similar de- 

 grees of coiling or involution appeared independently in distinct 

 genetic series, thus producing species which often closely re- 

 sembled each other in the adult type, but which had a wholly dis- 

 tinct ancestry. 



Among vertebrates many cases of similar independent morpho- 

 logical equivalents have been discovered, especially in the class of 

 mammals, and in that of reptiles. It sometimes happens that it is 

 impossible or impracticable to separate such heterogenetic homoeo- 

 morphs into their respective genera. In such cases a generic name 

 is retained for this polyphyletic group, but it must not be considered 

 a genus. The name Circulus (Bather) has come into use for such 

 groups of homoeomorphs. Platyceras is an example of a circulus 

 among gastropods ; it is commonly given the value of a genus, but 

 its species are phylogerontic terminals of a number of genetic 

 series. IMany of the generic names applied to graptolites, such as 

 Tetragraptus, Dichograptus, etc., are really circuli, for they repre- 

 sent similar stages in development in different genetic series. Since 

 they occur at the same geologic horizon they have also been called 

 "Geologic genera" (Ruedemann-39). 



A distinction has commonly been maintained between cases in 



