200 AN INTRODUCTION TO THE STUDY OF FOSSILS 



This has developed from Orthis lenticularis of the Upper 

 Cambrian, a pHcate species with a smooth median fold upon the 

 pedicle valve and a smooth sinus upon the brachial. Later the 

 median fold of the former becomes depressed between the two 

 bounding plications forming the central plication of the median 



Fig. 82. — Platystrophia lynx (Eichwald), from the Upper Cincinnatian of Indiana. 

 a, surface of pedicle valve, natural size ; a', junction of the two valves anteriorly ; 

 a", junction of the valves posteriorly, showing the open delthyrium ; b, exterior 

 of pedicle valve of a very young individual, showing the introduction of plica- 

 tions; b', posterior view of same; b.v., brachial valve; p.v., pedicle valve. 

 The natural size of h' is noted by the black spot to the right. (From 

 Cumings, who has worked out the recapitulation of Platystrophia.) 



sinus of the adult shell, while the two plications bounding 

 the early median sinus upon the brachial valve become, through 

 their enlargement, two of the plications of the strong median fold 

 of the adult. This shifting of the median sinus from brachial 

 to pedicle valve may be due to the enlargement of the median 

 lobe of the brachia, thus keeping the mantle from sagging here. 

 All species of Platystrophia, whether from the Ordovician or 

 Silurian, pass through the same Orthis lenticularis stage (shown 

 at the beak in well-preserved specimens). Platystrophia 

 looks much like Spirifer, but has no calcareous supports for its 

 brachia, and the cardinal area of the brachial valve is almost as 

 high as that of the pedicle valve. 



Platystrophia lynx is a very abundant species from the Lor- 

 raine (mid-Ordovician) of the Appalachian region and of the 

 Ohio Valley. 



