C. E. Beecher — Development of the Brachiopoda. 149 



early species may become accelerated, and pass into the nealogic 

 period of later forms, while other characters remain ephebolic. 

 Discinisca offers an illustration of this. Its nealogic characters 

 agree with Orbiculoidea in the form of the valves and in the 

 pedicle notch, but the circular or elliptical form of the dorsal 

 valve in adult and nealogic Orbiculoidea appears so early in 

 Discinisca, that it marks all the nepionic stages. The inter- 

 pretation of these facts is, of course, very evident, and will be 

 subsequently given in detail. Attention is here called to the 

 statement, that while nepionic, nealogic, and ephebolic stages 

 represent equal intervals in the life of each individual, they do 

 not represent conditions of growth, or the possession of 

 characters which always agree stage for stage in the species of 

 one family or of different families. 



Other distinctions to be made whenever possible are (a) 

 whether certain characters (natural or acquired) belong to a 

 species by inheritance, or (h) are mere adaptations to special 

 conditions of environment arising at any time in its history. 

 A clear understanding of the first will lead to the true 

 phylogeny of a species or genus, but to reach this the charac- 

 ters of the second category must be excluded. Thus in the 

 series of Schizocrania, Orbiculoidea, and Discinisca, already 

 cited, there is an apparent genetic connection in the facts as 

 stated. The contrary must be the case with a shell like Lingula 

 complanata Williams, and L. riciniformis Hall which initiate 

 a holoperipheral* mode of growth in the ephebolic period, for 

 this agreement in the method of concrescence with adult 

 Orbiculoidea here appears in the mature stages of this species, 

 and being absent in the early members of the genus cannot 

 therefore be an ancestral character. It is a morphological 

 equivalent, which may or may not be continued in the later 

 species of the series. ■ 



Whenever features are present which can be referred to an 

 ancestral origin, their elimination can take place only by the 

 process of acceleration of development. On the other hand, 

 there may be secondary characters of dynamical or homoplastic 

 origin which appear simultaneously or independently in dif- 

 ferent groups belonging to diverse genetic lines, as the 

 deltidial plates of the Rhynchonellidse, Terebratuliclse, and 

 Spiriferidse. Further, many such secondary features may 

 occur anywhere in the geological history of the group, as the 

 high hinge area of Orthisina, Spirifer, Syringothyris, and 

 Thecidium. These statements are in full accord with what 

 Hyatt has determined in the Cephalopoda, and the application 

 of such ideas affords a fertile field of research. 



* 6?i,dg, whole ; rrtpujiepeia, circumference. 



