394 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



It marks a development toward a loss of plications on the lateral 

 slopes, while PlatystropJiia cypha marks the culmination of a develop- 

 ment toward an increase in the length of the hinge line and in the num- 

 ber of plications on the lateral slopes. This is accompanied by the 

 development of a high compressed fold and profound smus on which 

 the lateral plications have disappeared or exist only as rudimentary 

 structures. These characteristics represent the liighest degree of 

 specialization reached by the high fold subgroup. The culmination 

 is followed by decline, which is indicated by the development of a 

 galaxy of variants. Those variants, which are tied together by a 

 group of constant characteristics, are described in the following pages 

 as varieties of tliis species but by far the greater number will permit 

 of no classification. 



In the Lower Maysville members of tliis subgroup were rare. 

 However, from late Fairmount until the end of the Maysville, they 

 form the most conspicuous fossil, being represented by great num- 

 bers of PlatystropJiia crassa in the Upper Fairmount, PlatystropJiia 

 laticosta, and PlatystropJiia unicostata in the Belle vue and Corryville 

 and Mount Auburn. At the end of the Maysville most of these spe- 

 cies became extinct. PlatystropJiia cypJia, however, lived into the 

 Arnheim and became associated with the members of the low fold 

 subgroup, wliich had migrated into the area. 



From a study of the early development of members of the Ponde- 

 rosa subgroup the conclusion was reached that these specimens repre- 

 sent degenerate branches of the genus which originated wliile the 

 race was still in a progressive state of development. 



PlatystropJiia preponderosa, new species, of the Upper Trenton 

 group, passes through a PlatystropJiia precursor development and 

 then becomes globose. There existed during the Maysville and 

 lower Richmond time a large form with a long hinge line and equi- 

 convex valves, and low spreading fold. The early development of 

 these is almost identical with that of PlatystropJiia precursor. Most 

 of these large PlatystropJiias are globose with the brachial valve much 

 the deeper, and with moderately elevated folds. 



The writer's investigation revealed very little as to the origin of 

 this protean group. All of them, however, pass through a low-old 

 development which indicates their origin from some member of that 

 subgroup. It may be that various members possessed the possi- 

 bility of large size wliich was realized in one or more lines. When 

 their energy was exhausted, decline was marked by assumption of 

 gerontic characters and the development of multitudinous forms 

 which are so diverse as to permit of little classification. 



PARALLELISM IN DEVELOPMENT. 



This genus furnishes several examples of convergence in develop- 

 ment. The adult members of the most specialized mutations of 



