148 0. E. Beecher — Development of the Brachiopoda. 



but points to the existence, in the early articulate genera, of 

 an anal opening dorsal to the axial line, as in the recent 

 Crania. This dorsal foramen was described and figured by 

 King 13 in 1850, Hall 8 in 1860 and by several authors since, 

 and has commonly been termed a visceral foramen. 



GEhlert' 21 suggests that it was probably occupied by the ter- 

 minal portion of the intestine. The persistence of the foramen 

 seems to indicate an anal opening. In reference to this 

 character and the obsolescence of the eyes the class must be 

 viewed as retrogressive since paleozoic time. Other features, 

 however, are manifestly progressive ; namely, the gradual 

 shortening, through time, of the posterior elements of the 

 animal, as the pedicle, visceral portions, and internal shell 

 structures, and the expansion of the anterior parts, as the shell 

 and brachia. 



A further advance in specialization is shown in the limitation 

 of the pedicle opening wholly to the ventral valve in the 

 higher rhynchonelloids, atlryroids, spiriferoids, and terebratu- 

 loids. The absence of punctse in all the early radicles and 

 their subsequent development in the derived types msty also 

 have a similar bearing. 



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 (b) the phylogenetical. The ontogeny of a 

 form like Schizocrania may be conveniently divided into the 

 nepionic, nealogic, and ephebolic periods, and such stages may 

 be clearly defined. The ephebolic stage of Schizocrania, how- 

 ever, is like a nealogic stage of Orbiculoidea. In other words, 

 Orbiculoidea, in its development, passes through a Schizocrania- 

 like stage before reaching maturity.* These facts must be 

 viewed from a phylogenetic standpoint. Moreover, in the 

 geological history of a group, certain ephebolic characters of 



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

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

 development of the pedicle opening in Orbiculoidea was fully described. 



