No. 490] ORTHOGENETIC VARIATION 



637 



parisiensis IMayer-Eymar (or better Clavellojusus spiratm) from 

 a common Eocene or earlier ancestor, without the intermediation, 

 in the Cyrtiilus line, of modern Fusus, his proceeding would be 

 almost as illogical as the classification of all ammonoids showing 

 a corresponding degree of involution in the same genus. This 

 will appear more clearly from a consideration of the following 

 diagram. 



Modem Fusus — Cyrtulus 

 Xeoccnc Fusus 



Eocene Fusus Clavilitlies 



I , 



Fusoid radicle 



The only other way, in which Clavilithes and Cptulus can be 

 made congeneric, is by also including Fusus in this genus, a 

 stretching of generic limits, to which even ^I. Cossmann will most 

 certainly object. 



1 have elsewhere^ oudined in detail a number of genetic series 

 among the Clavilithoids which, diverging probably from a com- 

 mon ancestor, produced similar end forms, just as divergent am- 

 monite phyla often had end forms superficially alike. Whether 

 the series which I have outlined to the degree of detail permitted 

 by the available material, will be found to be complete, or will 

 need modification in the future, remains to be seen. That the 

 various series exist, is, I think, beyond cavil, and certainly cannot 

 be set aside by a wholesale assertion of authority even on the part 

 of the most veteran collector of these fossils. 



It has already been outlined in the earlier part of this paper, that 

 old age characters in gastropod shells are also shown by the loss 

 of the power to coil, as is so commonly the case in cephalopods. 

 That such characters have been taken as distinctive of new genera 

 is not surprising, and indeed is desirable. Great caution however 

 is necessary not to make this the sole distinguishing character, and 

 class together terminal loose-coiling members of distinct genetic 



»Phylogeny of Fusus. Smithsonian Miscellaneous Collections, vol. XLIV, 

 no. 1417, 1904. 



