DEVELOPMENT OF THE BRACHIOPODA 269 



the features which reach their complete growth in the 

 adult organism are introduced and progressively developed. 

 Usually they appear in succession, and gradually assume 

 mature conditions. Thus in many species with radiate plica- 

 tions or strise, a few radii appear in early neanic 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, and 11, Plate XII, represent a neanic 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 characteristic adult features have made their 

 appearance. 



Ephebic 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 ephebic shells of the species 

 given are represented in figures 3, 6, 9, and 12, Plate 

 XII. 



Gerontic Period. — The variations due to old age may be 

 numerous and complex. As shown by Clarke and the 

 writer,'^ the valves generally become thickened, and, as a 

 consequence, the margins are truncate or varicose, the ver- 

 tical diameter of the shell is increased, the beaks involuted, 

 and the margins of the valves often lose the ornamentation 

 characteristic of the species. The deltidial plates or del- 

 tidium may be resorbed as well as the beaks of the valves. 

 Usually the ephebic characters disappear in inverse order to 

 their introduction. Thus in a normal adult brachiopod hav- 

 ing a plicate shell and deltidial plates, which characters were 

 introduced during the neanic period, the expression of old 

 age will be found in the absorption of the deltidial plates and 

 in the obsolescence of the plications. Large specimens of 

 Terehratella transversa Sowerby often furnish examples of 

 this condition. 



