278 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



The later Silurian is marked by a very docided development of the 

 genus Beyrichia and its related type, Kloedenia. A striking feature re- 

 specting these genera is that whereas the true species of Beyrichia are 

 exceedingly rare in all American deposits of similar ages, on the other 

 hand Kloedenia occurs on both sides of the Atlantic, while another genus, 

 Kloedenella, obtained extraordinary development in the Silurian rocks of 

 the Appalachian Valley and is almost unknown in Europe. True Lepev- 

 ditiidse continue throughout the Silurian. 



With the inauguration of the Devonian the general aspect of the Ostra- 

 coda changes markedly. True Leperditiidse have practically disappeared, 

 only a few stragglers occurring in the lower beds of the Helderbergian. 

 The Beyrichiidge have modified itno new generic groups with a quite 

 different aspect. The hitherto poorly represented genera, like Kirhbya, 

 Octonaria, Tlilipsura, etcetera, now make up a considerable proportion 

 of the total number. The general aspect of this ostracod fauna was not 

 materially changed until the close of the Paleozoic. In abundance and 

 variety American Devonian ostracods are in contrast with those of Europe 

 because the latter are so poorly developed. It appears that the area of de- 

 velopment and dispersal was again shifted back to South Atlantic waters. 

 In fact, it seems that they were almost confined to these waters until well 

 toward the close of the Paleozoic. In the Pennsylvanian a number of 

 types not hitherto seen are introduced, the notched cypridinoids, primi- 

 tive Cytheriidse, and numerous cytherelloids. At this time a host of 

 fresh-water forms are introduced — the first known. By this time the 

 marine Ostracoda. have become so cosmopolitan that the locus of their 

 development can no longer be traced. Although still of aid in broad 

 correlation, their value in detailed correlation and in the discrimination 

 of paleogeographic provinces has been almost entirely lost. In succeed- 

 ing time the fresh-water forms become more and more abundant. They 

 are frequently found in the Eed Beds of the West, and layers are often 

 almost made up of them in the land deposits of the Cretaceous and Ter- 

 tiary. While a few can be determined as land forms, many others are so 

 similar to the marine Cypridae that on their own evidence it would be 

 almost impossible to decide that they are actually land forms and not 

 marine. 



From the foregoing it will be seen that while the Ostracoda are of very 

 considerable value in stratigraphic correlation and throw light on paleo- 

 geographic problems up to the Pennsylvanian, their value in this respect 

 seems to be much diminished in later times. 



