NO. 2297. BRACHIOPOD GENUS PLAT78TR0PHIA—MCEWAN. 391 



gibbous, subquadrate forms with the hinge about equaling the 

 width; these represent the species described by Doctor Foerste as 

 Platystropliia ponderosa and seem to be confined to the Bellevue. 

 There are globose forms found in the Mount Auburn which Foerste 

 has described as Platystropliia ponderosa auhurnensis; and there is 

 an unusually large transverse form confined to the Arnheim forma- 

 tion which the writer is describing as Platystropliia ponderosa 

 arnlieimensis. But there are innumerable variants which refuse to 

 mark any definite horizon and before they can be of any use to 

 stratigraphy further data are needed. 



RELATION OF THE GROUPvS. 



As the uniplicate stage is common to all PlatystropMas, it is evi- 

 dent that there was a uniplicate ancestor with three and four pli- 

 cations on the lateral slopes. The species most closely related to 

 this ancestral form is Platystropliia uniplicata, new species, from the 

 New York Lower Trenton. That this is not a retrograde from the 

 triplicate group is shown by the fact that none of the many speci- 

 mens associated with it showed a tendency toward loss of the lateral 

 plications of the fold and sinus. There is a uniplicate species in the 

 Lower Trenton of Europe, but this form is gibbous and has a more 

 specialized fold and sinus. 



Divergence from the uniplicate stock must have taken place in 

 early Ordovician or Upper Cambrian time, as somewhat specialized 

 members of the triplicate group are found in the Black River. 



The biplicate group must have diverged at a very early stage in 

 the development of the genus, as the modification of the phcations 

 of the fold and sinus takes place before the shell has reached a length 

 of 1 mm. 



Subgroup A shows the least differentiation, as the median plication 

 has not appeared in the sinus, and the median plication of the fold 

 has not bifurcated. It therefore stands closer to the ancestral stock 

 than the other subgroups. Its occurrence with the uniplicate group 

 and subgroup B does not necessarily mean that differentiation was 

 taking place in Lower Trenton time, but rather that the uniplicate 

 type and subgroup A were representatives of the primitive stocks 

 which give rise to the other subgroups in early Ordovician time. 



Subgroups B and C diverged from the radical represented by 

 subgroup A about the same time ; one added a plication in a median 

 position in the sinus and the other added plications on the slopes. 

 A very little later subgroup D diverged from C by adding plications 

 on the lateral slopes. 



As the triplicate group retains the uniplicate condition until the 

 shell has reached a length of 1.2 to 1.5 mm., it must have diverged 

 from the ancestral stock much later than the biplicate group. 



