24 



CHESTER OSTRACODES OF ILLINOIS 



establishes the persistence of this genus 

 throughout a considerable period of time. 

 The common Chester Monoceratina agrees 

 closely in general structural features and 

 ornamentation with Roth's genotype from 

 the lower Pennsylvanian. 



Triceratina inconsueta 



(Croneis and Gutke) 



Plate 1, figures 4-8 



Pterocodella inconsueta Croneis and Gutke, 1939, 

 Bull. Denison Univ., Jour. Sci. Lab., vol. 34, 

 p. 40, pi. 1, figs. 7-9, Renault formation. 



Length, 0.52 mm; height, 0.25 mm; 

 thickness, 0.26 mm. 



Renault formation, locality 9, rare; Men- 

 ard formation, locality 20, rare. 



The specimen illustrated is a topotype 

 and is identical with the holotype, except for 

 the somewhat greater swelling above the ven- 

 tral lobe. This species resembles Monocera- 

 tina lewisi Harris and Lalicker from the 

 Fort Riley limestone in its similarly sharp- 

 ended ventral lobe and prominently 

 rounded posterior lobe. Its dorsal swelling 

 corresponds to the dorsal lobe of the latter 

 species but it differs in possessing a more 

 acuminate anterior end in lateral view and 

 in the presence of a tubercle in the postero- 

 dorsal angle. 



Family Bairdiidae Sars 



Genus Acratia Delo 



AcRATiA MUCRONATA Cooper, n. sp. 



Plate 1, figures 40-42 



Carapace long thin and smooth, lens-like 

 in outline; ends sharply pointed and low 

 (near ventral margin), the posterior end 

 produced downward ; hinge line straight or 

 very slightly curved in centro-dorsal region, 

 but not depressed as in A. tumida; articu- 

 lation on ventral margin broadly but slight- 

 ly curved ; surface smooth. 



Length, 0.98 mm; height, 0.39 mm; 

 thickness, 0.44 mm. 



Glen Dean formation, locality 1, rare. 



Acratia obtusa Cooper, n. sp. 

 Plate 1, figures 1-3 



Carapace large, elongate, thick and 

 smooth ; ends abruptly terminated without 

 the tapering characteristic of other species; 

 hinge straight, but not depressed; left 

 valve slightly overlaps the right around en- 

 tire margin except in the centro-dorsal por- 



tion ; posterior end slightly produced down- 

 ward. 



Length, 0.80 mm; height, 0.35 mm; 

 thickness, 0.38 mm. 



Paint Creek formation, locality 2, rare. 



Acratia tumida Cooper, n.sp. 

 Plate 1, figures 35-37 



Carapace large, oval or lens-shaped in 

 outline; thicker than wide, with largest 

 dimensions central ; centro-dorsal region 

 slightly flattened ; hinge line slightly de- 

 pressed and strongly curved ; left valve 

 overlaps entire free margin, with greatest 

 overlap ventral ; hinge line strongly curved 

 toward left in centro-dorsal portion; sur- 

 face smooth. 



Length, 1.10 mm; height, 0.58 mm; 

 thickness, 0.58 mm. 



Kinkaid formation, locality 3, common. 



Genus Bairdia McCoy 



Bairdia aculeata Cooper, n. sp. 



Plate 1, figures 47-48 



Bairdia sp. Jones and Kirkby, 1879, Quart. Jour. 

 Geol. Soc. London, vol. 35, pi. 32, figs. 7, 8, 

 Carboniferous of Great Britan. 



Carapace large, with extremely long and 

 straight postero-dorsal slope with inclination 

 of 30-35° starting forward of the middle of 

 the shell ; dorsal overlap very conspicuous, 

 that on venter much less so ; point low and 

 thin ; hinge sinuous in dorsal view ; greatest 

 height forward of center. 



Length, 1.10 mm; height, 0.51 mm; 

 thickness, 0.39 mm. 



Kinkaid formation, locality 3, common. 



B. aculeata differs from other described 

 species in the long postero-dorsal slope, oc- 

 cupying about three-fifths of the dorsal 

 margin. The nearest known described form 

 is the one figured 2iS Bairdia sp. by Jones 

 and Kirkby from Fife. 



Bairdia aequa Cooper, n. sp. 

 Plate 1, figures 21-22 



Carapace ovate, short and thick, with 

 prominent antero-dorsal shoulder produc- 

 ing abrupt break in the curvature at this 

 point ; anterior end rounded ; posterior end 

 short and blunt ; postero-dorsal slope about 

 60° ; overlap most prominent along venter; 

 hinge area depressed ; greatest height for- 

 ward of center; sides distinctly flattened 

 in dorsal view; surface appears finely pitted. 



