390 PROCEEDINOS OF THE NATIONAL MUSEUM. vol. 56. 



In subgroup D a plication is intercalated in a median position in 

 the sinus and a lateral plication is implanted on each of the slopes, 

 while on the fold all the plications bifurcate. 



3. Triplicate group. — The greater number of North American 

 species belong to the triplicate group. In this group the primary 

 plication remains unmodified tliroughout the life history. In the 

 early neanic stage a plication is implanted on each slope of the sinus, 

 the one on the right side appearing first. Contemporaneous with 

 this development the two primary plications of the fold bifurcate 

 and form four plications. 



The triplicate group was found to be separable into three subgroups 

 which the writer is designating, the low fold, the high fold, and the 

 Ponderosa subgroups. The first two subgroups are alike in possess- 

 ing a long hinge line relative to the height. By a comparison of the 

 growth stages both are found to pass through a similar development 

 in the nepionic and early neanic stages. In the low fold subgroup 

 the low rounded fold persists throughout its development and the 

 plications of the fold and sinus remain of nearly the same strength. 

 The second subgroup in the late neanic stage develops a high com- 

 pressed fold on which the lateral plications become weak and tend 

 to disappear, giving the fold an angular appearance. 



Some individuals and varieties belonging to species of the low 

 fold subgroup have slightly elevated folds, on which the lateral 

 plications are somewhat weakened. This may be an expression of 

 an innate tendency of the race to develop a high fold which was 

 somewhat realized when the organism was subjected to the proper 

 environmental stimulation. 



Those species and varieties which are characterized by large size 

 are placed in the Ponderosa subgroup. 



These large PlatystropJiias have been called Platysirophia lynx or 

 PlatystropJiia hiforata, according as they were interpreted by the 

 investigator. 



PlatystropJiia hiforata has been discussed in the preceding pages 

 and it has been shown that the term is of no specific value. 



In 1830 Eichwald * described Terelratula {PlatystropJiia) lynx as 

 follows: "Spirifer, cardine elongato, recto, vertice, utriusque valvae 

 prominulo, utraque valva sulcata, stratis singulis transversis ex 

 testae incremento exortes, numerosissimis margine dentato; media 

 parte unius valvae prominula, quadrisulcata, alterius vero parte 

 eadem exclavata, profunda," 



Eichwald's specimen was from the drift and was not figured. 

 The description is not specific enough to delineate a species and 

 accordingly is of no value. 



Several hundred specimens were before the writer and an endeavor 

 was made to find some logical classification of them. There are 



1 Skizze von Podolis, p. 202. 



