400 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56 



low spreading folds and shallow sinuses. The genus had expressed 

 its invariability in Cambrian or early Ordovician time by differen- 

 tiation of the plication pattern of the fold and sinus. This differ- 

 entiation was at that time only of specific value. These species, 

 however, became the source of lines of development which per- 

 sisted until the extinction of the race. Thus, while near the origin 

 of the genus differentiation was not pronounced, the variation 

 which achieved expression was of high taxonomic value. 



The uniplicate group is confined to the Lower Trenton of North 

 America and seems to occupy that horizon in Europe, where it is 

 associated with members of the biplicate and triplicate groups. 



In North America the biplicate group is not found in the Ordo- 

 vician above the Lower Trenton except in the Uppermost Richmond 

 of Anticosti Island. Its recurrence in North American early Silu- 

 rian points to its existence during ^liddle and Upper Trenton and 

 Cincinnatian time in an area which was cut off from North American 

 seas. 



The writer had access to about 200 specimens from the Baltic 

 province of Europe. These range in age from Lower Middle Ordo- 

 vician (ivuckers) into the Silurian. A uniplicate species was found 

 to occur in the Jewe, and one specimen was found which occurred 

 in the Upper Ordovician (FO. All the other species belonged to the 

 biplicate group. 



The inaccessibility of the literature made it impossible for the 

 writer to determine the limits of the European species and the group 

 to which they belong. The figures and descriptions by Davidson 

 and other authors help but little. Davidson's figures 15a, 17, 18, 

 19^ represent specimens of the biplicate type from the Caradoc. 

 The Wenlock species (figs. 11, 11a) are also of the biplicate type and 

 seem to be related to Platystro'pliia regularis Shaler. The abundance 

 of this group throughout European Ordovician and its limitation to 

 the Lower Trenton and early Silurian of North America suggest an 

 invasion from that area which was followed by extinction of the 

 invading forms. 



The Trenton members of the triplicate group belong to the low- 

 fold subgroup which is most closely related to the ancestral stock. 

 Members of this subgroup persisted into the Maysville, but by the 

 end of that time they became rare. A reincursion in Richmond time 

 brought in several species belonging to the low-fold subgi'oup which 

 are scarcely distinguishable from Trenton forms. Extreme speciali- 

 zation and extinction was reached in late Cincinnatian and early 

 Silurian time. 



The high-fold subgroup originated in early Maysville. It became 

 the predominant type in Middle Maysville to Middle Richmond 



I British Fossil Brachiopoda, vol. 1, pi. 28. 



