638 



THE AMERICAN NATURALIST [Vol. XLI 



series. That this has been done in Vermetiis, admits of little 

 doubt. The Mesozoic and Tertiary species of this group repre- 

 sent phylogerontic terminals of various genetic lines of turritel- 

 loids, becoming extinct in these periods, just as modern species 

 represent terminals of one or more lines of modern turritelloids. 

 Even if the species of turritelloids are considered as congeneric 

 throughout, the terminal vermetoids cannot be congeneric but 

 merely represent a stage in development. Each terminal group 

 would of necessity represent a distinct genus unless it were united 

 in one genus with its corresponding ancestral turritelloid, as in 

 the following diagram. 



Vermetus stage 



Turritella stage | \__ J j 



Thus if a, b, c, and d represent four diverging specific lines of Tur- 

 ritella and 1, 2, 3, and Jf the corresponding Vermetus form, these 

 latter could not be classed as one genus, Vermetus, unless a, h, c, 

 and d were also classed in the same genus. Each must be con- 

 sidered as a separate genus whether it has one or more species, but 

 1 and a, 2 and h, 3 and c, 4 and d, can each be classed as a dis- 

 tinct genus, with a turritelloid and a vermetoid species, or all 

 species of vermetoids may be classed as Turritella. 



This reasoning applies with equal force to the non-coiling forms 

 commonly classed as Platyceras, this term having the value of a 

 stage. A similar though very slight loosening of the last whorl is 

 made the basis for the separation of the genus Diastema from 

 Melania, although pathologic individuals of the latter sometimes 

 show an identical character (Fig. 19). That in the case of this 

 so-called genus we really deal with a stage in development, and 

 therefore with terminal members of different phyletic series, which 

 ought to be united with their respective IMelania ancestors instead 

 of being classed together, becomes more apparent as the detailed 

 study of these types progresses. 



In extrem.'lv arnvlcriUc^I scrips it often liappcns tliai the mode 

 of develoiHiH-iit ai.]H'ar^ to lir iwcrx-d, tli;' .specialized eliaracter 

 appearing first, and the U\ss speeialiiied later. I'hus in certain 



