Cuming s — Morphogenesis of Platystrophia. 



25 



to 15 ft. at Richmond, comes in a large and decidedly trans- 

 verse variety of P. lynx associated with Hebertella sinuata* 

 This association is a point of great importance since it shows 

 that these two forms of Platystrophia and Hebertella, so con- 

 stantly met with together in the Lorraine, have lived on together 

 at some point through the Rhynchotrema hemeraf and here 

 under a recurrence of suitable conditions reappear, the former 

 with some modification, the latter with scarcely any.J 



This form of JPlaty atrophia lynx has eight to eleven plica- 

 tions on the lateral slopes, and the index may be as great as 1*9. 



11 



Fig. 11. A, Platystrophia lynx from 12 ft. below the Clinton at Richmond, 

 Indiana ; dorsal valve showing obsolescence of both lateral plications toward 

 the anterior margin. B, dorsal valve of another individual from the same 

 horizon and locality showing complete reduction of the fourth plication. C, 

 specimen of P. acutilirata from the extreme Upper Ordovician at Weisburg, 

 Indiana, showing the similarity of this retrogressive form to P. lynx. Author's 

 collection. 



A number of individuals (5 out of 100) show a reduction of 

 the lateral plications of the fold and sinus. Fig. 11, A, shows 

 a reduction of both lateral plications of the fold. On the 

 anterior portion of the fold, the one to the right has completely 

 vanished and the one to the left is very faint and does not 

 reach the margin. All four plications are about normal on 

 the posterior portion of the fold. Another specimen, fig. 11, 

 B, has three plications on the fold. The only indication of 

 the fourth is the abnormal breadth of the right hand plication 

 at the umbonal region. 



That such a tendency to eliminate plications should affect 

 5 individuals out of 100, while in the Lorraine not more than 

 one P. lynx out of a thousand exhibits anything analogous, is 

 certainly not without profound significance. There are in the 



* These beds are exposed at Richmond near the point where the " Boston 

 Pike" crosses Elkhorn Creek, S.E. of the city. The ledge of Clinton 12 ft. 

 thick forms a fall. Immediately below the Clinton is 4 ft. of clay and below 

 this, limestone layers alternating with shale. PlatystroipMa lynx is found in 

 large numbers in the limestone for some distance down the creek. 



f Buckman's term, or some other equally unequivocal, is needed in a case 

 like this where the lifetime of a species is referred to. See Quar. Jour. 

 Geol. Soc, xlix, 1893, p. 481. 



\ The Rhynchotrema zone does not contain any specimens of Hebertella 

 sinuata except at its very base. 



