Vol I. Grabau — Ordocician Fossils from North China (i) 19 



Weller descrilxul similar shells as abundant in the Ki-su-ling limestone on tlie 

 Ta-ning River in eastern Bzechuan, where tlioy were collected h}' Blackwelder. His 

 specimens indicate a somewhat larger size than our form, l.)Ut appear to he of the same 

 species. The reference of our species to the American S. rncurvata is tentative, and 

 indeed is probably not warranted. Better material is however necessary before it is 

 possible to make a complete characterization of our form. It A\'ill probably be found to lie 

 a new species. 



S. incitrrata is a characteristic and abundant fossil of the Trenton limestone of 

 New York and of the central United States, where it is widely distributed. 



Class PELECYPODA 

 Order Prionodesmacea 



Family CTENODONTID>E Hall 



Genus CTENODONTA Salter 

 Ctenodonta symmetrica Grabau (sp. nov.) 



Plate I Fig. 7. 



Sliell small, transverse, beak subcentral; hinge-line somewhat sharply arcuate, 

 marked with rather strong transverse denticulations ; ends subangularly rounded ; ventral 

 margin nearly straight. The anterior end is somewhat broader than the posterior, but the 

 difference is not very pronounced. The anterior muscle-scar also appears slightly larger 

 than the posterior, but again the difference is slight. It is however more strongly 

 excavated than the posterior. There is a rather strong though low rounded ridge, extend- 

 ing from the posterior ventral margin of the anterior adductor scar towards the rostral 

 cavity, dying away before it reaches this. Surface characters unknown, but apparently 

 the shell is marked by simple growth-lines. 



This small shell has some resemblance to Ctcnodonta logani Salter of the Black 

 River of Wisconsin and Ontario, but the ends are more nearly equal and the denticulated 

 hinge-line is more arcuate. The muscular impressions also are more pronounced, while 

 the short internal anterior muscular ridge is distinctive. From C. fccanda Hall of the 

 Upper Ordovician (iMaquoketa shale) it differs in the straight ventral margin, and the 

 strong muscular impri'ssions. It has the character of hinge, and of muscular impressions 

 of C. pectionculoides Hall, of the Cincinnati group, but is of less rounded form. 



