196 AN INTRODUCTION TO THE STUDY OF FOSSILS 



ably by the extension of the ventral mantle. Brachial valve 

 flat or concave. Hinge line straight. Surface with radiating 



pdm. 



Fig. 77. — Rafinesquina alternata Conrad (X f), from the Ordovician of Cincin- 

 nati, Ohio, yl, exterior of brachial valve. 5, exterior of pedicle valve. C, inte- 

 rior of pedicle valve of Strophomena. a.m., adductor muscle scar; a.d.m., pos- 

 terior diductor muscle scar; delt., deltidium ; p.d.m., anterior diductor muscle 

 scar; t., tooth. (From Hall and Clarke.) 



striae alternating in size. Shell punctate. (Name in honor of 

 the French-American scientist, Rafinesque.) 



R. alternata is the most widely spread and abundant species, 

 being found almost everywhere in the Middle to Upper Ordo- 

 vician of North America. 



1. Sketch (a) exterior of pedicle valve, (b) cardinal areas of 

 entire shell, (c) the cut portion of a vertical section through 

 entire shell from beak forward. Label in all, where present, 

 pedicle and brachial valves, hinge line, cardinal areas, beak, 

 deltidium, front of shell. 



2. Indicate in sketch i (a) the oldest portion of shell, stating 

 your reasons. 



3. How do you know the convex valve is the pedicle valve ? 



4. Draw in your sketch of the pedicle valve the outline of the 

 shell when half grown, stating how you know that this was its 

 shape. 



5. State whether the animal was attached or free when adult, 

 giving reasons. 



Strophomena (Figs. 77, C, 78). Ordovician. 



A resupinate Rafinesquina, i.e. pedicle valve concave or 

 sigmoid in section, and the brachial convex. The muscular 

 area on the pedicle valve is sharply limited by elevated ridges. 

 Shell punctate. 



