C. K Beecher — Development of the Brachiopoda. 151 



than to any telotremate genus. Until the early forms belong- 

 ing to the articulate brachiopods, especially to the orthoid and 

 strophomenoid groups, have been thoroughly studied, the 

 interpretation of the nepionic Beptcena rhomboidalis may be 

 uncertain. It should be noted, however, that the young of 

 Chonetes, Productus, Stropheodonta, Orthothetes, Leptsena, 

 Plectambonites, and Strophomena, all have little or no indi- 

 cation of a straight hinge line, and that the extension of this 

 member takes place during later nealogic and ephebolic growth. 

 This in itself is significant, but is more marked when taken 

 with the growth stages shown by some species of Strophomena 

 which have after the protegulum, a Paterina-like stage, with 

 straight hinge in dorsal valve, succeeded by holoperipheral, 

 discmoid, nepionic growth, and finally a renewal of a straight 

 hinged condition. Thus it has an early straight hinged form, 

 which is lost during the next stage of growth, and again 

 appears, and is progressively elongated daring nealogic and 

 ephebolic growth. 



The nepionic stages of Terebratulina septentrionalis, fig. 10 

 PI. I, represent a decreasing extension of the cardinal line from 

 the protegulum, an open delthyrium, the absence of radii, and 

 the introduction of the shell punctEe. The crura at this stage, 

 as shown by Morse, are short and stout, and the loop is unde- 

 veloped. 



Nealogic period. — During the progress of this period all the 

 features which reach their complete growth in the adult 

 organism are introduced and progressively developed. Usu- 

 ally they appear in succession, and gradually assume mature 

 conditions. Thus in many species with radiate plications or 

 strise, a few radii appear in early nealogic growth, and are 

 added to until the full number is present. Species with 

 deltidial plates develop them in this period. The early stages 

 may offer many points for comparison with the adult, but later 

 stages usually differ little except in size. Figures 2, 5, 8, 11, 

 Plate I, represent a nealogic stage in each of the four species 

 taken as examples. Others from the same species could be 

 given, but these suffice to show that one or more character- 

 istic adult features have made their appearance. 



Ephebolic period. — The period of complete normal growth, 

 or the maximum of individual perfection. This corresponds 

 to the adult, or mature organism, and is so well understood 

 that no further explanation is necessary. For the sake of 

 completing the series, the ephebolic shells of the species given 

 are represented in figures 3, 6, 9, 12, Plate I. 



Geratologic period. — The variations due to old age may be 

 numerous and complex. As shown by Clarke and the writer, 3 

 the valves generally become thickened, and, as a consequence, 



