THE MODIFICATION OF GENERIC CHARACTERS. 279 



Schuchert, characterized by the possession of a calcareous loop for the 

 support of the brachia.* 



Three Families of the Helicopegmata, — In the classification 

 of the Helicopegmata into famiHes Mr. Schuchert's simple 

 classification into the Atrypidae, Spiri- 

 feridae, and Athyridae, based upon the 

 essential structure of the brachidium, 

 is adopted. His definitions are: 



1. In AtrypidiC the primary lam- 

 ellae are directly continuous with the 

 crura, diverge widely, and have the 

 spirals between them (Fig. 63). 



2. In the Spirifcridce "ih.^ primary 

 lamellae are also directly continuous 

 with the crura, but lie between the 

 spirals, thus holding a position the reverse of that in the 

 AtrypidcB (Fig. 64). 



3. In the AtJiyridcB the primary lamellae differ in direc- 



Fig. 63 — The brachidium of the 

 Atrypidae ; Zygospira modesta, 

 enlarged ; view of interior from 

 the side of brachial valve, which 

 has been removed. (After Hall.) 



Fig. 64. Fig. 65. 



Fig. 64. — The brachidium of the Spiriferidse, Uncites gryphus Defr. ; interior of brachial valve, 

 viewed from pedicle-valve side. 



Fig. 65. — The brachidium of the Athyridae, Rhynchospira evax, enlarged, and viewed from the 

 pedicle-valve side. (After Hall.) 



tion from those in the other families in being more or less 

 sharply recurved dorsally near their junction with the crura 



(Fig. 65).t 



* Schuchert, "A Classification of the Brachiopoda," Am. Geol., vol. xi. 

 14T-167, 1093. 



t Schuchert, "A Revised Classification of the Spire-bearing Brachiopoda,'" 

 Ant. Geologis , vol. xiii. p. 102, 1894. 



