28 



CHESTER OSTRACODES OF ILLINOIS 



curved ridges, if extended would form flat 

 or elongate ellipse ; hinge depressed and 

 central; in dorsal view articulation forms 

 almost symmetrical reverse or S curve, 

 center of which is concave toward the right 

 valve; beak long, almost on center line. 



Length, 0.91 mm; height, 0.35 mm; 

 thickness, 0.28 mm. 



Renault formation, locality 13, common. 



Bairdiolites vulgaris Cooper, n. sp. 

 Plate 2, figures 41-42 



Carapace thick, overlap moderate around 

 entire margin ; hinge depressed, postero- 

 central; postero-dorsal slope steep (ca. 

 55°); venter straight in central portion 

 swinging upward to meet beak and an- 

 terior extremity ; antero-ventral curve forms 

 angle at each extremity, ventral articulation 

 swinging upward resulting in concave lower 

 margin on right valve ; beak of average 

 length, subcentral; greatest height central. 



Length, 0.84 mm; height, 0.48 mm; 

 thickness, 0.35 mm. 



Paint Creek formation, locality 2, rare. 



B. vulgaris may be distinguished by the 

 concavity of the ventral margin of the right 

 valve. 



Genus Bythocypris Brady 

 Bythocypris amsdenensis Morey 

 Plate 2, figures 35-36 

 Bythocypris amsdenensis Morey. 1935, Jour. Pale- 

 ontology, vol. 9, p. 481, pi. 54, figs. 2, 3, Ams- 

 den formation. 



Carapace subovate ; dorsal margin arched, 

 venter almost straight; overlap prominent 

 except on posterior end; greatest thickness 

 in posterior half, tapering anteriorly. 



Length, 0.48 mm; height, 0.29 mm; 

 thickness, 0.23 mm. 



Clore formation, locality 26, rare; Fay- 

 etteville shale, locality 31, abundant. 



Bythocypris clorensis 

 Croneis and Funkhouser 

 Plate 3, figures 1-2 

 Bythocypris clorensis Croneis and Funkhouser, 

 1938, Bull. Denison Univ., Jour. Sci. Lab., 

 vol. 33, p. 357, pi. 10, figs. 11, 12, Clore for- 

 mation. 



Bythocypris ulli Coryell and Johnson, 1939. Jour. 

 Paleontology, vol. 13, p. 222, pi. 26, figs. 13a, 

 b, Clore formation. 



Length, 0.48 mm; height, 0.31 mm; 

 thickness, 0.21 mm. 



Clore formation, locality 27, common ; 

 Kinkaid formation, locality 6, rare. 



Bythocypris concava Cooper, n. sp. 



Plate 3 figures 3-6 



Carapace elongate ovate, dorsum arched, 

 venter concave, posterior slope straight, 

 rising at high angle; overlap around entire 

 margin, most prominent along anterior end 

 and venter; hinge straight, surface smooth. 



Length, 0.46 mm; height, 0.23 mm; 

 thickness, 0.18 mm. 



Vienna formation, locality 10, rare; 

 Menard formation, locality 7, common; 

 Kinkaid formation, locality 6, common. 



B. concava, characterized by the high 

 angle of the postero-lateral slope and the 

 concave venter is very close to B. cuneola 

 Jones and Kirkby, 1886, which however, 

 has a straight venter. 



Bythocypris fabalis Cooper, n. sp. 

 Plate 3, figures 29-30 



Carapace large, bean-shaped, tumid ; ends 

 and dorsum rounded, venter almost straight 

 (slightly concave) ; greatest height post- 

 erior, greatest thickness well above middle; 

 greatest length near venter ; middle overlap 

 prominent; surface smooth. 



Length, 1.37 mm; height, 0.90 mm; 

 thickness, 0.60 mm. 



Paint Creek formation, locality 23, depth 

 530-540 feet, common ; Golconda formation, 

 locality 22, rare. 



Bythocypris gibba Cooper, n. sp. 



Plate 3, figures 7-8 



Carapace somewhat rectangular, ventral 

 extremities angular and produced into 

 sharp, thin edges; posterior margin almost 

 straight and nearly vertical ; dorsum broad- 

 ly curved, venter distinctly concave, sub- 

 parallel with dorsum; overlap prominent 

 except on venter ; greatest height and thick- 

 ness central, greatest length just above ven- 

 ter; surface finely granulose to smooth. 



Length, 0.53 mm; height, 0.25 mm; 

 thickness, 0.21 mm. 



Renault formation, locality 9, rare. 



Bythocypris modica Cooper, n. sp. 



Plate 3, figures 9-10 

 Carapace small, very ovate or subelllptl- 

 cal ; ends and dorsum rounded, venter 

 straight ; greatest height just back of middle ; 



