270 STUDIES IN EVOLUTION 



The gerontic development of Bilohites^ consists in the 

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

 shell, thus reverting to an early neanio condition equallj- 

 characteristic of B. hilohus and B. Verneuiliamis. 



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 gerontic and 

 pathologic forms. The oldest known shell with calcareous 

 spires, Zygospira, is a comparatively minute form. Nearly 

 all the types of the sub-order to which this genus belongs 

 (Helicopegmata) appear in the Upper Silurian. Species pre- 

 senting the maximum size belong to the Devonian and 

 Carboniferous. Before the extinction of the sub-order in the 

 Trias, the individuals are small, and such abnormal genera as 

 Thecospira, KonincMna, and Amphiclina abound. Productus 

 begins with small species {Productella) in the Lower Devo- 

 nian, and in the Carboniferous attains the largest dimen- 

 sions of any known brachiopod (P. giganteus). During 

 the Permian the species have dwindled in size, and the 

 gerontic Strophalosia and Aulosteges are the chief repre- 

 sentatives. 



The culmination of gerontic growth results in the rever- 

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

 the paragerontio 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 gerontic heredity. 

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

 nounced paragerontic type. The shell has a small internal 

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

 crural plates. King,^* the author of the genus, states that 



* 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 cistellula, and C. neapolitana. This question cannot be at present deter- 

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



