770 



CHALMER L. COOPER 



Cavellina, Amphissites, and possibly Bair- 

 dia, many of which later make important 

 contributions to Carboniferous faunas, make 

 their appearances. 



After the Devonian, the whole aspect of 

 the Lower Paleozoic Ostracoda changed so 

 that few of the old familiar ones remain. The 

 Beyrichia, Ctenobolbina, Isochilina, Leper- 

 ditia, and most of the Primitia have disap- 

 peared. The place of dominance in the Car- 

 boniferous is occupied by such genera as 

 Hollinella, Sansabella, Jonesina, Semino- 

 lites, Healdia, Glyptopleura, and Cytherella 

 {ly in addition to those mentioned above. 

 Many reach their culmination in the Upper 

 Pennsylvanian and carry over into the 

 Permian. A few, such as Bairdia, Pontocy- 

 pris, Bythocypris, Macrocypris and Cyther- 

 ella carry on into the Recent. 



The geologic ranges of the Paleozoic os- 

 tracode genera are shown graphically in fig- 

 ures 8 and 9. 



INDEX OSTRACODES 



Ordovician. — As might be expected, the 

 oldest known ostracode horizon, the Beek- 

 mantown, contains relatively few species. 

 Isochilina dominates the list, with Leper- 

 ditia, Primitia, and Entomidella following in 

 order. They are rather common in the Stones 

 River of Kentucky and Tennessee, and the 

 Chazy of New York, and they become in- 

 creasingly abundant throughout the Mo- 

 hawkian in the Black River of Kentucky 

 and Minnesota, and in the Trenton of Ohio, 

 Kentucky, Tennessee, and New York. The 

 upper Lowville has a fauna of ostracodes 

 which extends from New York to Alabama, 

 thence to the Mississippi Yalley States and 

 northward through Canada."* The shale- 

 limestone formations of the Cincinnatian 

 have furnished a large number of species, 

 especially in the upper part of the series in 

 the Richmond beds of Ohio and Indiana. 



Some Middle Ordovician genera are Apa- 

 tochilina, Dicranella, Hallatia, Monocera- 

 tina, Parabolbina, Rayella, Saccelatia, and 

 Winchellatia. 



^ The occurrence of this common post-Paleo- 

 zoic genus in the Carboniferous is questioned be- 

 cause of the close similarity to Cavellina, which 

 ranges from the Devonian to the Permian. Future 

 work may show that the Paleozoic species of 

 Cytherella should be classified with this genus. 



* Bassler, R. S., personal communication. 



Cincinnatian genera include Barychilina, 

 Biflabellum, Jonesella, and Milleratia. 



Silurian. — Many of the Ordovician gen- 

 era continue into the Silurian, some of the 

 persistent types increasing in range and 

 others declining, becoming extinct by the 

 end of the period. The various divisions of 

 the Cayugan, as well as the Rochester shale 

 and its equivalents, can be traced through- 

 out the Appalachian Valley by their ostra- 

 code zones. ^ 



The Lower Silurian is very poorly defined 

 in ostracode faunas, but the Jupiter River 

 beds of Anticosti are characterized by a 

 number of species of Zygobolba in addition to 

 a few species of Apatobolbina, Chilobolbina 

 and Leperditia. Other localities yield numer- 

 ous species of Aparchites, Beyrichia, Bollia, 

 and Bythocypris. Leperditia and Primitia 

 both showed marked decreases in the Lower 

 Silurian. 



The Middle Silurian, the Clinton of 

 Maryland and Pennsylvania is one of the 

 few formations that has been completely 

 zoned by means of these fossils. Ulrich and 

 Bassler (1923) have described nine zones, 

 each characterized by the prolific occur- 

 rence of a different ostracode species as 

 shown on figure 2. These zones have been 

 recognized at a number of widely separated 

 localities in New York, Tennessee, Ohio and 

 Indiana. 



The Upper Silurian (Cayugan series) con- 

 tains many horizons with an abundance of 

 ostracodes, particularly in the McKenzie 

 and Wills Creek formations of Maryland 

 and Pennsylvania. Few genera are restricted 

 to the series, although several reach the cul- 

 mination of their development, with very 

 few species in formations above and below. 

 Some of these are Dizygopleura, Eukloede- 

 nella, Kloedenella, and Zygobeyrichia. 



The development of Bollia, Bythocypris, 

 Entomis, Kloedenia, and Octonaria contin- 

 ues more or less regularly into the Lower 

 Devonian. Beyrichia, Dizygopleura, and Eu- 

 kloedenella have few or no representatives 

 after the end of the Silurian. Leperditia has 

 reached its peak and declines rapidly in the 

 Devonian. 



Devonian. — The rapid decline of many of 

 the characteristic stocks of the Lower Paleo- 

 zoic at the end of the Silurian reaches com- 

 pletion in the Devonian. At the same time 



