32 



CHESTER OSTRACODES OF ILLINOIS 



termination of the posterior end, particul- 

 arly in the dosal view. 



Healdia ornata Morey 

 Plate 4, figures 1-2 



Healdia ornata Morey, 1935, Jour. Paleontology, 

 vol. 9, p. 481, pi. 54, fig. 4, Amsden forma- 

 tion. 



Healdia triangularis Croneis and Gale, 1938, 

 Bull. Denison Univ., Jour. Sci. Lab., vol. 33, 

 p. 290, pi. 6, fig. 22, Golconda formation. 



Length, 0.48 mm; height, 0.31 mm; 

 thickness, 0.21 mm. 



Glen Dean formation, locality 11, 

 common. 



The Amsden and Illinois specimens agree 

 very closely in general outline, in the de- 

 velopment of the sharp keel-like posterior 

 transverse ridge which is perpendicular to 

 the venter, and in the symmetry of the dorsal 

 arch. 



Healdia ovoidea Cooper, n. sp. 

 Plate 4, figures 15-17 



Carapace ovate, dorsum and venter 

 curved, ends rounded ; overlap conspicuous ; 

 rib straight, inclined slightly backward and 

 curving forward at ends to merge with shell 

 near upper and lower margins; surface 

 smooth. 



Length, 0.48 mm; height, 0.29 mm; 

 thickness, 0.22 mm. 



Renault formation, locality 9, abundant ; 

 Golconda formation, locality 19, common. 



Healdia radinula Cooper, n. sp. 

 Plate 4, figures 18-20 



Carapace elongate, anterior tapering to 

 rounded end, dorsal and ventral margins 

 broadly curved ; rib almost straight in 

 central portion, curving broadly to meet 

 dorsum and sharply to meet venter; over- 

 lap moderate, surface smooth; greatest 

 height and thickness postmedial. 



Length, 0.62 mm; height, 0.35 mm; 

 thickness, 0.30 mm. 



Paint Creek formation, locality 24, 

 common. 



H. radijiula is distinguished from H. 

 menisca by the tapering of the anterior half 

 of the carapace. 



Healdia tenuicosta Cooper, n. sp. 

 Plate 4, figures 13-14 



Carapace somewhat elongate, symmetri- 

 cal; dorsum arched, venter slightly convex. 



ends equally rounded; rib thin, strongly 

 curved, located well back from end ; postero- 

 dorsal slope flat, articulation channelled ; 

 overlap slight; greatest height central. 



Length, 0.51 mm; height, 0.28 mm; 

 thickness, 0.23 mm. 



Vienna formation, locality 10, rare; 

 Menard formation, locality 7, rare; Clore 

 formation, locality 4, abundant; Kinkaid 

 formation, locality 5, common. 



Healdia vinitaensis Harlton 



Plate 4, figures 11-12 



Healdia ^vinitaensis Harlton, 1929, Am. Jour. Sci., 

 ser. 5, vol. 18, p. 262, pi. 2, figs, la, b, Fay- 

 etteville shale. 



Length, 0.60 mm ; height, 0.32 mm ; 

 thickness, 0.22 mm. 



Vienna formation, locality 10, common. 



The Illinois and Oklahoma specimens are 

 identical in form ratio and shape of outline. 

 There may be some question about the 

 agreement of the posterior ridges or nodes, 

 due to the lack of clearness in the illustra- 

 tion of Harlton's specimen. 



Genus Incisurella Cooper, n. gen. 



Carapace thick, subovate outline; dorsal 

 margin arched, ventral margin slightly con- 

 vex ; ends rounded ; overlap moderate around 

 entire margin except on postero-dorsal slope, 

 where the articulation is a narrow, slit-like 

 channel and the slope of this portion of the 

 shell resembles Healdia; subovate shallow 

 area just in front of posterior margin im- 

 pressed into the shell, interrupting but not 

 changing its regular curvature ; overlap left 

 over right. 



Genotype. — /. prima n. sp. 



This genus possesses many features of 

 Healdia but differs in the development of 

 the depressed area on the posterior end. 



Incisurella prima Cooper, n. sp. 

 Plate 4, figures 27-29 



Carapace thick subovate, ends rounded, 

 dorsum arched, venter almost straight, but 

 slightly convex; ventral margin of right 

 valve somewhat concave; postero-dorsal 

 slope short, articulation along hinge slit- 

 like ; overlap moderate ; depression near 

 posterior end flat, ovate slightly subcentral ; 

 surface smooth, greatest thickness and 

 height just back of center. 



