198 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Stropheodonta (Fig. 80). Silurian to Devonian. 



Shell similar to the preceding but with the hinge margins 



marked by more or less long transverse ridges which terminate 



in teeth that articulate as teeth and 

 sockets (whence the name from Greek 

 stropkis, a band, + odous, a tooth, a 

 band of teeth). Shell punctate. 



a.a.m. 



p. am. 



Fig. 80. — Stropheodonta concava 

 Hall, from the Hamilton (De- 

 vonian) of New York. ( X f .) 

 Muscle area of the interior of 

 a brachial valve, a. a.m., the 

 broad posterior adductor mus- 

 cle scars; c.p., cardinal pro- 

 cess for attachment of di- 

 ductor muscle; gr., vertical 

 grooving, or ridging, of the 

 edge of shell at hinge line, 

 giving rise to the name ; 

 p.a.m., the elongate anterior 

 adductor muscle scar. (From 

 Hall and Clarke.) 



1. Sketch (a) exterior of pedicle 

 valve, (b) cardinal areas of entire shell. 

 Label valves, deltidium, transverse 

 ridges. 



2. How do you distinguish this 

 genus from Rafinesquina ? 



times produced into ears. 



Productus (Fig. 81). 



Devonian to Permian. 

 Shell free, or anchored in the mud 

 by the tubular spines growing from 

 the convex pedicle valve. Brachial 

 valve concave, spinose or lamellose. 

 Hinge line not well developed, some- 

 Entire surface marked with radiating 

 ribs, or elongate pustules, which are usually studded with spines. 

 Shell punctate. (Name from Latin productus, produced ; so 

 named from the prolongation of the beak of the pedicle valve 

 beyond that of the brachial.) 



Productus semireticulatus is world-wide in distribution, in 

 the Mississippian and Pennsylvanian. 



1. Looking fully upon the hinge line of entire shell, sketch a 

 view showing brachial valve and umbo of pedicle valve ; indicate 

 valves, spines, radiating ribs. 



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

 giving reasons. 



3. How were the spines formed? 



4. Is Productus semireticulatus a good index fossil ? 

 Reasons. 



