398 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



An examination of the diagram shows that all Platystrophias pass 

 through stages A, B, C, and it is inferred that these stages represent 

 the entire hfe history of a primitive ancestor which hved in late 

 Cambrian or early Ordovician time. The uniplicate group adds 

 stage D, and the more specialized species of the group add stages 

 F and I. Subgroup A recapitulates the development of the unspe- 

 cialized members of the uniphcate group and adds stage E, and the 

 more specialized species add stages F and I. Subgroup B recapitu- 

 lates the ontogeny of the unspecialized members of subgroup A and 

 adds stage E* and the more specialized species add F and I. Sub- 

 group C recapitulates the ontogeny of the unspecialized members of 

 subgroup A, and adds stage E^ and the more specialized members 

 add F and I. Subgroup D recapitulates the ontogeny of the unspec- 

 cialized members of subgroup B and adds stage E^ and the special- 

 ized species add stages F and I. 



The low-fold subgroup recapitulates the ontogeny of the unspe- 

 cialized members of the uniplicate group and adds stage E^ and 

 the more specialized species adds stages F and I. The Ponderosa 

 subgroup recapitulates the ontogeny of the low-fold subgroup and 

 adds stage H, and the high-fold subgroup recapitulates the ontogeny 

 of the low-fold subgroup and adds stage E. 



CORRELATION OF THE PHYLOGENY AND STRATIGRAPflKJ SUCCESSION. 



This progressive development is in harmony with the stratigraphic 

 succession of the species representing the groups and subgroups. 



According to our present knowledge species of this genus appeared 

 in North America and Europe in early Middle Ordovician time. 

 The earliest North American form is found in the Black River 

 group (Decorah Shale) 4§ miles north of Fennemore, Tennessee. 

 This form has been referred to Plafysirophia extensa, new species, 

 and belongs to the triplicate group. As the long hinge and broad 

 lateral slopes bearing numerous plications indicate a certain degree 

 of specialization it is evident that the genus was in existence prior 

 to this time. 



Hall and Clarke ' have reported a form from the Chazy. Winchell 

 and Schuchert say,^ "This widely distributed and protean species 

 has its beginning in North America in the Chazy group and is found 

 in all geological horizons upward and into the Niagara formation." 

 Schuchert ^ and Ruedemann * have also reported occurrence of 

 species in the Chazy. 



' Paleontology of New York, vol. 8, pt. 1, Brachiopoda, 1892, p. 202. 

 ' Geol. of Minnesota, vol. 3, pt. 1, Paleontology, 1892, p. 456. 

 » Synopsis of American Fossil Brachiopoda, 1897, p. 309. 

 « Bull. N. Y. State Museum, No. 49, 1891, 1902, p. 25. 



