Branson — The Fauna of the Residuary Auburn Chert. 41 



ventral margin. Beaks prominent, recurved, situated 

 anterior to center of shell. A sharply defined anterior 

 cardinal sulcus extending from beaks to antero-ventral 

 margin. Anterior to the beaks the hinge plate is long, 

 slightly convex, provided with about eighteen short, 

 strong, slightly curved or straight teeth. Teeth small 

 near beaks. Posterior to beaks the hinge plate is shorter, 

 straight, provided with about twelve teeth. These teeth 

 are smaller than the largest teeth anterior to beaks. Ven- 

 tral margin of shell nearly semicircular, convexity 

 slightly increased where it passes into the posterior 

 hinge line. Surface of shell marked with about eighteen 

 strong concentric ridges. These are smallest and closest 

 together near the beaks, strongest on middle third of 

 shell. 



C. costaia resembles C. similis Ulrich in shape. Its 

 long anterior sulcus resembles that of C. recurva Ulrich. 

 It ditf ers from these forms in the concentric ridges, being 

 much more prominent and closer together and having no 

 finer striae between, in the number of teeth being smaller, 

 and the beaks being anterior to the middle of the shell. 



This species is based on twelve well preserved speci- 

 mens, one of which shows the dentition. Number 11503 

 of the invertebrate paleontological collection of Walker 

 Museum. 



MODIOLODON SUBKHOMBOIDEUS, U. Sp. 



PI. vii. f. 1. 



Shell small, subrhomboidal, height five to six milli- 

 meters, length along the hinge line three to four milli- 

 meters. Valves gibbous near the beaks. Posterior cardi- 

 nal region compressed; beaks prominent, slightly in- 

 curved ; hinge plate very thin, two small cardinal teeth in 

 left valve, one small tooth at the posterior end of the 

 hinge plate parallel with the hinge line. Anterior to the 

 beaks is a small projection separated from them by an 

 indistinct sulcus. The rest of the anterior margin of the 



