34 



CHESTER OSTRACODES OF ILLINOIS 



anterior and dorsum; posterior ridge thin, 

 but quite high, directed backward along 

 dorsal margin ; anterior ridge indistinct, 

 little more than slight indentation and fold 

 in regular curvature of shell ; hinge line 

 sinuous; surface smooth. 



Length, 0.60 mm; height, 0.37 mm; 

 thickness, 0.25 mm, 



Renault formation, locality 13, rare. 



Seminolites? reversus Cooper, n. sp. 

 Plate 4, figures 41-42 



Carapace subovate laterally, ends round- 

 ed, posterior more acute, dorsum arched; 

 overlap right over left around entire margin, 

 most pronounced along dorsum and venter ; 

 greatest height and length through center; 

 greatest thickness posterior ; parenthesis-like 

 ridges at each end, subparallel to margin ; 

 anterior half higher than posterior. 



Length, 0.72 mm ; height, 0.47 mm ; 

 thickness, 0.33 mm. 



Golconda formation, locality 17, rare. 



The lateral outline of this species is 

 almost the exact reversal of S. compressus 

 Coryell and, considering the thicker end 

 to be posterior, the overlap is also reversed. 

 There are other notable differences how^- 

 ever which mark S. reversus as a new species, 

 namely, the marked thickening of the over- 

 lap near the mid-portions of the dorsal and 

 ventral margins and the lack of large 

 punctae. 



Seminolites symmetricus Cooper, n. sp. 

 Plate 4, figures 53-55 



Carapace subtriangular, tumid, ends 

 rounded, dorsum arched, venter almost 

 straight, but slightly convex; moderate 

 overlap around entire margin except 

 posterior slope of dorsum, w^hich is flattened 

 much as in Healdia, articulation of this 

 portion slightly channeled; posterior ridges 

 thin, but prominent, subparallel to margin; 

 anterior ridge short, low, and parallel to 

 margin, inner side depressed Into slight 

 furrow ; greatest length and height central ; 

 surface smooth. 



Length, 0.55 mm; height, 0.33 mm; 

 thickness, 0.27 mm. 



Paint Creek formation, locality 2, 

 common. 



This species may be distinguished by Its 

 symmetrical lateral outline, slight overlap, 

 and the character of the posterior ridge. 



Genus Macrocypris Brady 



Macrocypris acuminata Cooper, n. sp. 



Plate 4, figures 30-31 



Carapace elongate, ends rounded, dorsum 

 arched, venter straight; overlap most prom- 

 inent on posterior end, anterior narrow and 

 tapering to well rounded point ; greatest 

 thickness and height about middle of 

 posterior half ; greatest length below 

 middle ; surface smooth. 



Length, 0.72 mm; height, 0.41 mm; 

 thickness, 0.33 mm. 



Menard formation, locality 30, rare. 



Macrocypris ovata Cooper, n. sp. 



Plate 4, figures 25-26 



Carapace elongate, elliptical, ends 

 rounded, dorsum arched, venter concave; 

 greatest thickness near mid-length; overlap 

 prominent along dorsum and venter only. 



Length, 1.18 mm; height, 0.53 mm; 

 thickness, 0.36 mm. 



KInkaid formation, locality 3, common. 



Genus Tetratylus Cooper, n. gen. 



Carapace ovate, ends rounded, dorsum 

 curved, venter straight to convex, end 

 margins of some species bordered by low 

 ridge, terminated above and below by 

 round, knoblike spines of variable length ; 

 valves highest anteriorly, with shallow sinus 

 just back of center which is elongate verti- 

 cally, extending from the dorsum down to 

 about one-third of shell height, deepest near 

 bottom ; right valve overlaps left around 

 entire margin, but overlap Is inconspicuous 

 except along venter; surface smooth to 

 finely punctate. 



Genotype — T. elUpticus, n. sp., Paint 

 Creek formation. 



Tetratylus differs from Seminolites 

 In that the ridge, when present, is on 

 and not back of the end margins of the 

 shell, in the presence of four spines or knobs, 

 and the presence of a sulcus; from Healdia 

 In the presence of anterior spines and 

 sulcus ; and from Cribrospira n. gen. in 

 the presence of the anterior spines and 

 ridge, the sulcus, and the lack of circular 

 pits. 



The family classification of Tetratylus 

 is somewhat uncertain because of the pri- 

 mitlan sulcus and the cytherelloid outline. 



