THE MODIFICATION 1 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 families 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 Atrypidce the primary lam- 

 ellae are directly continuous with the 

 crura, diverge widely, and have the 

 spirals between them (Fig. 63). 



2. In the Spiriferida the primary FIG. 6 3 -The jbrachidium of the 

 lamellae are also directly continuous 



with the crura, but lie between the 



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



Atrypidce (Fig. 64). 



3. In the Athyridcs the primary lamellae differ in direo 



. 



Atrypidae ; Zygospira modesta^ 

 enlarged ; view of interior from 

 the side of brachial valve, which 

 has been removed. (After Hall.) 



FIG. 64. FIG. 65. 



hidium of the Spiriferidae, Uncites gryphus Defr. ; intern 

 dicle-valve side. 



FIG. 65. The brachidium of the Athyridae, Rhynchospira evax, enlarged, and viewed from the- 



FIG. 64. The brachidium of the Spiriferidae, Uncites gryphus Defr. ; interior of brachial valve r 

 viewed from pedicle-valve side. 



. 5. e racum o e t 

 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. 6 5 ).f 



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

 141-167, 1093. 



t Schuchert, "A Revised Classification of the Spire-bearing Brachiopoda/* 

 Am. Geologis , vol. xm. p. 102, 1894. 



