DEVELOPMENT OF THE BRACHIOPODA 265 



The features and importance of the protegulum have pre- 

 viously been discussed. 1 It is merely noticed here as the 

 embryonic shell of the completed phylembryonic period, for 

 it is the first stage which can be observed among the fossil 

 species, and is the initial point for the discussions of the 

 relations and affinities of recent and fossil forms. Of the 

 protegulum and later stages, there is abundant material avail- 

 able in nearly every family of brachiopods, ranging through 

 their entire geological history. 



Post-embryonic Stages. 



In discussing the post-embryonic stages of growth two 

 aspects of development must be clearly differentiated ; (a) the 

 ontogenetical, and (6) the phylogenetical. The ontogeny of 

 a form like Schizocrania may be conveniently divided into 

 the nepionic, neanic, and ephebic periods, and such stages 

 may be clearly defined. The ephebic stage of Schizocrania, 

 however, is like a neanic stage of Orbiculoidea. In other 

 words, Orbiculoidea, in its development, passes through a 

 fSchizocrania-like stage before reaching maturity.* These 

 facts must be viewed from a phylogenetic standpoint. More- 

 over, in the geological history of a group, certain ephebic 

 characters of early species may become accelerated, and pass 

 into the neanic period of later forms, while other characters 

 remain ephebic. Discinisca offers an illustration of this. 

 Its neanic characters agree with Orbiculoidea in the form of 

 the valves and in the pedicle-notch, but the circular or ellip- 

 tical form of the dorsal valve in adult and neanic Orbicu- 

 loidea appears so early in Discinisca that it marks all the 

 nepionic stages. The interpretation 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, neanic, and ephebic stages represent equal intervals 



* Attention was called to this fact in a publication preliminary to vol. viii of 

 the Palaeontology of New York, pp. 131, 132, issued February, 1890. Also, the 

 development of the pedicle-opening in Orbiculoidea was fully described. 



