CHESTER OSTRACODES OF ILLINOIS 



contain marine fossils at a few localities. 

 The}' also are quite variable in character 

 and usually can be identified only by their 

 association with the adjacent limestone- 

 shale formations. 



With the exception of the Aux Vases, all 

 New Design and Romberg formations thin 

 to the west, the Bethel being represented by 

 a cherty horizon in outcrops near Missis- 

 sippi River. There the Renault and Paint 

 Creek are clastic littoral deposits, but con- 

 tain considerable limestone to the east be- 

 yond the limits of the Aux Vases. Neither 

 the Cypress nor the Hardinsburg is well 

 developed in the west, the latter being pres- 

 ent as a cherty zone in the Okaw formation. 



The upper Chester, the Elvira group, 

 seems in general more uniform than the 

 lower groups, although the Vienna, Wal- 

 tersburg, and Palestine thin both east and 

 west from a central area in Johnson and 

 Pope counties. The Degonia thins to the east, 

 seemingly to compensate for the thickening 

 of the Clore in this direction. The be- 

 havior of the Kinkaid is not very well 

 known, as its thickness is quite variable 

 due to pre-Pennsylvanian erosion. The other 

 formations of the Elvira thin to the west. 

 Many of the Chester sands thin and dis- 

 appear, and the limestone-shale formations 

 thin and become more dominantly shale to 

 the southeast in west-central Kentucky, 

 where the entire Elvira grades into a single 

 shale unit, the Leitchfield formation. To 

 the north in Indiana, however, the Chester 

 sandstones are all present, though thinner 

 and more shaly than in the central part of 

 the basin. 



Ostracodes are abundant in some calca- 

 reous shale beds of all of the Chester lime- 

 stone-shale formations. The number of spe- 

 cies recognized in each formation is roughly 

 proportional to the number of collections 

 studied. Although the greatest number of 

 species, 94, is recorded from the Golconda 

 (7 collections) and the smallest number, 40, 

 from the Glen Dean (2 collections), it is 

 believed that continued study will maintain 

 about the same proportion of species shown 

 in the check list on page 8. 



Like the larger invertebrate fossils a 

 greater number of diagnostic species occur 

 in the lower than in the higher formations. 

 In the New Design group 50 per cent of the 

 species are, so far as now known, restricted 



to one formation, whereas in the Romberg 

 and Elvira groups only 39 and 30 per cent 

 respectively are so restricted. 



The greatest change in ostracode faunas 

 appears to occur at the Homberg-Elvira 

 boundary. Only seven species are known 

 to be present in both the Glen Dean and 

 Vienna formations, although 21 of the 40 

 Glen Dean species occur in the Golconda, 

 and of the 48 Vienna species 32 occur in the 

 Menard. This relationship may be some- 

 what exaggerated because of the relatively 

 small number of Glen Dean and Vienna 

 species that have been recognized, but it 

 seems unlikely that additional material 

 would entirely eliminate this discrepancy. 

 In contrast the New Design - Homberg 

 boundary is not so clearly marked by change 

 in the ostracode faunas. Eighteen species 

 are common to the Paint Creek and Gol- 

 conda, whereas 15 of the 61 Paint Creek 

 species occur in the Renault and 20 of the 

 94 Golconda species are represented in the 

 Glen Dean. The following table shows sim- 

 ilar relationships for all of the Chester 

 formations. 



Sixty-seven genera of ostracodes have been 

 recognized in this study. Of this number 

 27 are, as far as known, restricted to the 

 Mississippian system, 11 range from the 

 Devonian to the Pennsylvanian, seven oc- 

 cur in both the Devonian and Mississippian, 

 and 22 are present in both the Mississip- 

 pian and Pennsylvanian. Genera alone are 

 of little or no stratigraphic significance. 

 Only eight genera are known from but a 

 single formation and all of these are rare 

 forms, five of them being known from only 

 one species each. Additional collections will 

 undoubtedly extend most of their ranges. 



Over half of the known Chester species, 

 however, appear to be restricted to a single 

 formation. This does not mean, however, 

 that these are all good index fossils. The 

 distinguishing characteristics of the species 

 in several genera are so slightly different 

 that, even though usually found in great 

 abundance, they are easily confused and 

 therefore are of little value for correlation. 

 Falling within this category are Bairdia, 

 CavelUna, Healdia, and a number of the 

 species of the old Amphissitinae, many of 

 which are included in the new genera Ecto- 

 de?nites and Polytylites. 



