I 'nl I. <lralin Ordovician Fossils from North China (l) 17 



plications (or of the interspaces) is characteristic of a distinct evolutional direction, and 

 the increase in number is equally characteristic of another, the two evolutional lines 

 1 icing divergent. Interpreted in terms of mantle growth, which is, after all, the key to the 

 surface features of the shell, the regular increase in si/c of the plications without inter- 

 calation of new striae, signifies a uniform interstitial growth of the marginal or shell- 

 building mantle-tissues, i. c. a uniform rate, and a uniform distribution of growth. The 

 development of intercalated plications on the other hand, signifies a proportionately more 

 rapid mantle growth of the portions corresponding to the interspaces, and this excess of 

 growth has to be compensated for by a folding of the mantle, and the corresponding 

 formation of plications. In other words, individuals with intercalated plications indicate 

 unequal mantle growth, while those in which new plications are added only at the hinge- 

 margin, indicate relative uniformity of mantle growth. It is evident, that the two groups 

 represent divergent lines of evolution. Our variety belongs to the latter group, and it is 

 the one figured and described by de Verneuil as variety orthambonites, and it is to his 

 figures 8b - d, that our form most nearly corresponds. 



The variety described by Weller from eastern Szechuan has fine radiating striae 

 intercalated between the coarser ones, while that described and figured by Kayser from 

 the "Light gray Brachiopod limestone of Kiau-tchang-pa, " though larger, agrees in all 

 essentials with the specimens from Chihli. 



HORIZON AND LOCALITY: In the upper part of the Machiakou (Actinoceras) 

 limestone of Chaokouchuang in the Kaiping coal basin, eastern Chihli province. CoJlec- 

 ted by survey expedition. 



Family STROPHOMENIDAE King 



Genus STROPHOMENA Rafinesque 

 Strophomena cf incur vata (Shepard) 



Plate I fig. 6. 



cf.1893 Strophomena inciirrata (Shepard), Winchell and Schuchert. Paleontology of Minnesota, V<1. 



I, (With bibliography and synonymy.) 

 cf.1909 Stropliomcna inciirrata (Shepard), Grabnu and Shinier, North American Index Fossils, Vol. 



I, p. 223, figs. 271, a- d. 

 cf.1913 Strophomena sp. undt. Weller, in Willis. Research in China, Vol. Ill, p. 281, pi. 25, figs. 1, 2. 



