24 



C timings — Morphogenesis of Platystrophia. 



10, <?, is drawn from an individual which departs very little 

 from the normal form of the Lorraine.* Fig. 10, b, is an aver- 

 age specimen from Trenton Falls, N. Y., enlarged to the same 

 size. The two shells have the same index, nearly the same 

 number of plications, and differ little in contour. The cardi- 

 nal angle of the Lorraine specimen is 89°, that of the Trenton 

 Falls specimen 96°. The latter is more gibbous. It would 



Fig. 10. Platystrophia lynx, ad , normal Lorraine type from Vevay, 

 Indiana, natural size ; 66', normal Trenton type from Trenton Falls, N. Y.; 

 x 2%. a, author's collection ; 6, collection of Mr. Wm. L. Porter. 



be possible to select plent}^ of adult individuals from the Tren- 

 ton and Lorraine that except for size would present no appre- 

 ciable differences. The slight change of the average shell 

 index (from 1*4 in the Trenton specimens to 1*3 in the Lor- 

 raine (Vevay) specimens) is due solely to the more robust 

 growth of the latter, which always have the higher index in 

 early ephebic stages (see fig. 13). It will be shown later that 

 the extreme manifestation of this tendency produces in the 

 upper Lorraine a shell of very low index; while the latest 

 representatives of lynx, now to be described, have again a very 

 high index. 



In the extreme upper part of the Ordovician (Madison beds) 

 of Richmond, Weisburg and Laurel, Indiana, the writer found 

 a variety of Platystrophia lynx which is of exceptional inter- 

 est (fig. 11). As is well known, the so-called Richmond beds 

 ( Rhynchotrema zone J contain exclusively the variety acutili- 

 rata associated almost constantly with Hebertella occidentalism 

 At about 50 ft. below the Clinton at Laurel and about 13 ft. 



* This specimen is from Vevay, Indiana. 



