ONTOGENY AND PHYLOGENY IN BRACHIOPODA 289 



simpler genera. It is evident that in the identification of 

 species belonging to the Terebratellidae, whether recent or 

 fossil, the strict specific characters must be given first con- 

 sideration. 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 

 individuals evidently fully adult. 



In each line of progression in the Terebratellidae, the 

 acceleration of the period of reproduction, by the influence of 

 environment, threw off genera which did not go through the 

 complete series of metamorphoses, but are otherwise fully 

 adult and even may show reversional tendencies due to old 

 age ; so that nearly every stage passed through by the higher 

 genera has a fixed representative in a lower genus. More- 

 over the lower genera are not merely equivalent to, or in 

 exact parallelism with, the early stages of the higher, but 

 they express a permanent type of structure, as far as these 

 genera are concerned, and after reaching maturity do not 

 show a tendency to attain higher phases of development, but 

 thicken the shell and cardinal process, absorb the deltidial 

 plates, and exhibit all the evidences of senility. 



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