304 STUDIES IN EVOLUTION 



form and eutelliform stages are larval, and the platidiform 

 represents an adolescent condition. Similar comparisons may 

 be made in the other genera. Progressively through each 

 series the adult structure of any genus forms the last imma- 

 ture stage of the next higher, until the highest member in 

 its ontogeny represents serially, in its stages of growth, all 

 the adult structures, with the larval and immature stages of 

 the simpler genera. 



Conclusions. 



It is impossible, with present knowledge, to go deeply into 

 the chronological history of the genera of the Terebratellidse. 

 They certainly appeared before Jurassic time, because they 

 were then well represented by several characteristic genera ; 

 namely, Kingena, Ismeyiia, Zellania^ and Megathyris^ al- 

 though some of them may represent incomplete develop- 

 ments of higher forms. Also, as among recent species, 

 several separate generic and specific names may have been 

 given to stages of growth of a few species. It is evident that, 

 in the identification of specimens in this family, whether re- 

 cent or fossil, the strict specific characters must be given first 

 consideration. Species, therefore, must be based upon surface 

 ornaments, form, and color, within certain limits, and genera 

 only upon structural features developed through a definite 

 series of changes, the results of which are permanent in indi- 

 viduals evidently fully adult. 



The austral distribution of the Magellaniinse and the boreal 

 distribution of the Dallinina3 have been emphasized already, 

 but, as has been stated, this difference in geographical posi- 

 tion in itself does not necessarily constitute a basis for sub- 

 family separation. The facts of develoi:)ment, however, and 

 the distribution of the genera show that the radical stock 

 was dispersed probably in Mesozoic time, and since then 

 evolution has gone forward through different lines of progres- 

 sion, and has terminated in similar types of structure. 



In each line of progression the acceleration of the period 



