262 NORTH AMERICAN INDEX FOSSILS. 



convex near the hinge with an undefined depression along the 

 median line which broadens rapidly anteriorly. Striae very fine, 

 numerous and irregularly bifurcating. 



Fig. 314. Dalmanella subcarinata. (After Hall.) 



Helderbergian of New York, Tennessee, Missouri, Wiscon- 

 sin, Nova Scotia. 



LVI. Rhipidomella Oehlert. 



Shell almost circular. Both valves gently convex. Hinge line 

 short. Slight median depression present in each valve. Surface 

 covered with fine, hollow striae, often opening on the surface. 

 From the base of the two strong teeth on the interior of the pedicle 

 valve a curving ridge extends forward and borders the large flabel- 

 liform muscular scars which extend one third or more the length 

 of the valve and are deeply impressed. Shell structure punctate. 

 Siluric-Carbonic. 



A. Rounded posteriorly, i. e., beak but slightly elevated above hinge line *. 



* Large (average width over ^ inch) I. 



I. With broad medial depression in pedicle valve a. 



a. Outline usually transversely oval i / . 



i / '. Posterior border curved 195. K. oblata. 



i / . Posterior border straight 196. R. aha. 



a. Outline circular 198. R. vanuxemi. 



1. Without broad depression in pedicle valve b. 



b. Beaks prominent 197. R.livia. 



b. Beaks not prominent 2 / . 



2 / . Radiating striae indistinct 202. R. michelini. 



2 / . Radiating striae strong 200. R. penelope. 



* Small (width not exceeding ]/ z inch) 2. 



2. Sinus present in both valves 201. R.thiemei. 



2. Sinus absent in pedicle valve 205. R. pecosi. 



