152 G. E. Beecher — Development of the Brachiqpoda. 



the margins are truncate or varicose, the vertical 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 deltidium ma} r be resorbed as 

 well as the beaks of the valves. Usually the ephebolic charac- 

 ters disappear in inverse order to their introduction. This is 

 called the clinologic stage of geratology by Hyatt. Thus in a 

 normal adult brachiopod having a plicate shell and deltidial 

 plates, which characters were introduced during the nealogic 

 period, the expression of old age will be found in the absorp- 

 tion of the deltidial plates and in the obsolescence of the 

 plications. Large specimens of Terebratella transversa Sow. 

 often furnish examples of this clinologic stage. 



The geratologic development of Bilobites 2 consists in the 

 obsolescence, in B. various Con., of the bilobed form of the 

 shell, thus reverting to an early nealogic condition equally 

 characteristic of B. bilobus and B. Verneuilianus. 



Another aspect of growth and decline is manifest when the 

 size of individuals and the chronological history of groups are 

 taken into consideration. Each genus and family began with 

 small representatives, and rapidly developed the more radical 

 varieties of structure. Then came the culmination and final 

 reduction in size, with abundance of geratologousand pathologic 

 forms. The oldest known shell with calcareous spires, Zygo- 

 spira, is a comparatively minute form. Nearly all the types of 

 the suborder to which this genus belongs (Helicopegmata) appear 

 in the Upper Silurian. Species presenting the maximum size 

 belong to the Devonian and Carboniferous. Before the extinc- 

 tion of the suborder in the Trias, the individuals are small, and 

 such abnormal genera as Thecospira, Koninckina, and Amphi- 

 clina, abound. Productus begins with small species (Produc- 

 tella) in the Lower Devonian, and in the Carboniferous attains 

 the largest dimensions of any known brachiopod {P. giganteus). 

 During the Permian, the species have dwindled in size, and 

 the geratologous Strophalosia and Aulosteges are the chief 

 representatives. 



The culmination of geratologous growth results in the re- 

 version of the animal to its own nepionic period, and is called 

 the nostologic stage. As this is an extreme condition, it can 

 be found only in certain genera and species which have been 

 developed by a process of accelerated geratologous heredity. 

 If Gwynia* is accepted as a valid genus, it belongs to a pro- 

 nounced nostologic type. The shell has a small internal plate 

 on each side of the dorsal umbo, evidently the bases of crural 



*Some authors have been disposed to consider this form as the young of a 

 species not yet determined. It has also been referred to Macandrevia cranium, 

 Cistella cisiellula, and ft neapolHana. This question cannot at present be deter- 

 mined, although some characters of the shell indicate a mature organism. 



