NO. 2297. BRACHIOPOD GENUS PLATYSTROPHIA—McEWAN. 395 



Platystro'plhia crassa and PJutystropMa unicostata crassiformis, new 

 variety, are narrow globose forms with high-compressed folds and 

 deep sinuses on which the lateral plications are weak or have dis- 

 appeared. The gTowth stages and the many variants shov/ that the 

 former is derived from Platystro'pMa pmiici'plicata while the latter is 

 derived from the laticosta stock. 



Tyi^ical specimens of the adult PlatystropMa unicostata of the 

 Maysville group find their almost exact equivalent in adult specimens 

 of PlatystropMa cypha heUatula, new variety, of the middle Richmond, 

 although the Ma3^s^ ille form originated from PlatystropMa laticosta 

 and the Richmond form from PlatystropMa cyplm. 



A comparison of the biplicate and triplicate groups furnishes a good 

 example of parallel development. Belonging to both are retrograde 

 globose species. There are trans% erse species with high-compressed 

 folds and deep sinuses on which the plications are rudimentar}^ or 

 absent. There are species in both groups with extremely long hinge 

 lines and many })lications on the lateral slopes, and there are large 

 ponderosa-like forms. 



This seems to indicate that the ancestral species had certain latent 

 possibilities, and though divergence took place early, these possi- 

 bilities were transmitted to the various groups and subgroups and 

 were expressed in a definite order whenever the appropriate environ- 

 mental stimulation was present. 



PHYLOGERONTIC TYPES. 



PlatystropMa rhynchonelliformis, new species, of the Lower Trenton; 

 PlatystropMa amoena rohusta, new variety, of the Middle Trenton; 

 and PlatystropMa glohossa, new species, and PlatystropMa prepon- 

 derosa. new species, of the Upper Trenton are globose forms which 

 show a tendency to loss of plications next to the cardinal angles. 

 They are phylogerontic members of the genus which have developed 

 while the race was still young. 



The Ponderosa subgToup, which is abundantly represented in the 

 Maysville group of Indiana, Ohio, and Kentucky, seems to represent 

 the phylogerontic end members of lines from the low fold subgroup, 

 which soon exhausted their energy in assuming large size. Professor 

 Cumings has called it a ^eri table race of gerontic individuals. The 

 greater number of them have the thickness equalling or exceeding 

 the width and there is obsolescence of plications next to the cardinal 

 angles. An excessive deposit of lime occui-s in all of them. These 

 characters point to loss of vigor and mark areturn to second childhood. 



Another example of phylogerontism is found in PlatystropMa 

 crassa. PlatystropMa crassa passes through a long-hinged develop- 

 ment and in the late neanic stage takes on gerontic characteristics. 

 The globose tendency becomes so pronounced in this species that the 

 thickness equals or exceeds the height in the adult. Increase in 



