MICROFAUNA FROM JOHNSON COUNTY, ILLINOIS 



85 



little from A. mar ginij eras Roth from the 

 Union Valley and Wapanucka formations of 

 Oklahoma. 



Glyptopleurina iniqua (pi. 22, fig. 35) and 

 Hollinella radiata (Jones and Kirkby) (figs. 

 20-23) are typical, easily recognized, and 

 range no higher than the Chester. However, 

 without close examination Glyptopleura 

 similis Croneis and Funkhouser (figs. 32-34) 

 might be confused quite easily with G. 

 coryelli Harlton which has a long range in 

 the Pennsylvanian. The species of Healdia 

 are characteristically Chester. 



Other good Chester markers, are Glypto- 

 pleura tyri Coryell and Johnson (pi. 23, figs. 

 9-11), Perprimitia funkhouseri Croneis and 

 Thurman, (figs. 15-17), Polytylites quincol- 

 linus Harlton (fig. 25), and Sargentina crassi- 

 marginata Cooper (figs. 30-34). However, 

 the reversed forms of Sansabella harrisi 

 Croneis and Funkhouser (figs. 26-29) might 

 easily be mistaken for S. carbonaria Cooper 

 from the Pennsylvanian. 



In summary, the fauna described is from 

 the youngest Mississippi an beds in the 

 type area, which can be studied in connec- 

 tion with the Mississippian-Pennsylvanian 

 contact. While predominantly Chester, the 

 microfauna reflects other Chester faunal 

 and sedimentological characteristics in that 

 it heralds the approaching Pennsylvanian 

 by showing a distinct break from the Middle 

 Mississippian, while at the same time con- 

 taining some early Pennsylvanian elements. 

 All of the fauna, with the exception of the 

 foraminifer Trepeilopsis and the ostracodc 

 Hollinella radiata show a complete lack of 

 Middle or Lower Mississippian affinity, 

 while most of the Foraminifera, including 

 the fusuline Millerella, and some ostracodes 

 and conodonts have a distinctly Pennsyl- 

 vanian character. 



SYSTEMATIC DESCRIPTIONS 



Order Foraminifera d'Orbigny, 1826 



Family Fusulinidae M oiler, 1878 

 Genus Millerella Thompson, 1942 



Millerella Thompson, 1942, Am. Jour. Sci., vol. 

 240, no. 6, p. 404.— Thompson, 1944, Kansas 

 Geol. Survey, Bull. 52, p 4i9 — Thompson, 

 1945, idem., Bull. 60, p. 40. 



Shell disc-shaped, periphery angular to 



rounded, usually completely planispiral, but 

 some species may have an endothyroid 

 juvenarium while others have only slight 

 irregularities in coiling; volutions 4 to 7, 

 completely involute to slightly evolute; 

 evolute species depressed or concave in polar 

 area (umbilicate) and convex in involute 

 forms; diameter one-fourth to one-half 

 millimeter, axis usually one-fourth to one- 

 third the diameter; spirotheca thin and 

 believed to possess three walls, septa thin 

 and long to short and thick (A-shaped); 

 proloculum very minute; tunnel and 

 chomata usually well developed. 



The above generic description is essen- 

 tially that of Thompson (1942, p. 404) but 

 modified to include many subsequently 

 described species, including those from the 

 Kinkaid. The latter are entirely consistent 

 generically with the Pennsylvanian forms 

 and differ only in those minor structural 

 features which are usually considered spe- 

 cific. 



Millerella chesterensis Cooper, n. sp. 

 Plate 19, figures 1-5 



Shell very small, discoidal, periphery 

 rounded; juvenarium endothyroid, essen- 

 tially planispiral; spirotheca and septa 

 thick, the latter almost A-shaped in section, 

 essentially straight, but directed forward 

 making an angle of about 65°, on the 

 average, with spirotheca; coiling elongate 

 about juvenarium, resulting in an elongate 

 peripheral outline in sagittal section; last 

 volution partially evolute, forming a slightly 

 umbilicate shell, the degree of concavity 

 being unequal on opposite sides. 



Data on shell measurements and septal 

 counts is shown in table 1. 



M. chesterensis differs from M. marblen- 

 sis Thompson in its smaller size, shorter, 

 thicker septa, larger tunnel angle, rounded 

 periphery (see axial sections) and is almost 

 completely involute. M. inflecta Thompson, 

 although quite similar in axial section has 

 thinner spirotheca and septa, greater regu- 

 larity of coiling, a nonendothyroid juven- 

 arium, and larger proloculum, diameter and 

 form ratio. 



Kinkaid formation, bed 5, JohnsonCounty, 

 Illinois. 



