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 suleus 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. costata resembles C. similis Ulrich in shape. Its 
long anterior suleus resembles that of C. recurva Ulrich. 
It differs 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 SUBRHOMBOIDEUS, N. sp. 
Pi oe Ff. I. 
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 
