38 



CHESTER OSTRACODES OF ILLINOIS 



somewhat indistinct; greatest height and 

 thickness just back of center, greatest length 

 central ; hinge straight. 



Length, 0.76 mm; height, 0.55 mm; 

 thickness, 0.38 mm. 



Menard formation, locality 15, rare. 



P. ovata resembles P. opima n. sp. from 

 the Paint Creek, but is larger, not so 

 abruptly terminated anteriorly, and has 

 its greatest thickness more nearly central. 



Paracavellina pinguis Cooper, n. sp. 

 Plate 6, figures 7-8 



Carapace short, thick ; outline subellipti- 

 cal in lateral view; ventral margin almost 

 straight, dorsum arched ; anterior ridge 

 more prominent than posterior; surface 

 smooth. 



Length, 0.95 mm ; height, 0.62 mm ; 

 thickness, 0.47 mm. 



Golconda formation, locality 17, common. 



P. pinguis may be distinguished from the 

 genotype by the shorter, thicker carapace 

 and by the relatively greater width across 

 the anterior and posterior ends in lateral 

 view. 



Paracavellina tumida Cooper, n. sp. 

 Plate 6, figures 17-18 



Carapace ovate, tumid ; right valve much 

 thicker than left; greatest thickness poster- 

 ior ; terminal ridges only slightly developed, 

 posterior one quite indistinct ; hinge irregul- 

 ar or sinuous; dorsum arched, venter less 

 so; overlap greatest along venter; greatest 

 thickness and height just back of middle, 

 greatest length central ; surface smooth to 

 pitted just inside terminal ridges. 



Length, 0.74 mm; height, 0.50 mm; 

 thickness, 0.39 mm. 



Paint Creek formation, locality 24, 

 common. 



Genus Platychilus Cooper, n. gen. 



Carapace cytherelloid in outline; ends 

 rounded, overlap inconspicuous at these 

 points ; dorsum and venter strongly arched, 

 overlap very prominent ; valves regularly 

 convex or with a broad shallow depression 

 near mid-dorsal area; hingement cardine^ ; 

 surface smooth. 



sThe ''tongue and groove" articulation at the hinge, 

 common in so many ostracodes, is, unfortunately, often 

 described as denticulation. Since the "tongue" does not 

 possess the usual shape of a denticle or tooth, it is pro- 

 posed to use the word "cardine" (L. cardinis, a pivot 

 and socket on which the doors of the ancients revolved, 

 or m building, a tenon and mortise) for this type of 

 ostracode hingement. 



Genotype. — Platychilus ovoides n. sp., 

 Vienna formation. 



Platychilus resembles Cavellina somewhat 

 closely but may be readily distinguished 

 from it by the unusual development of the 

 overlap along the midportions of the dorsal 

 and ventral margins, especially by the latter. 

 It also lacks the post-ventral truncation 

 common in Cavellina. 



Platychilus ovoides Cooper, n. sp. 



Plate 6, figures 26-27 



Carapace cytherelloid in outline ; ends 

 about equally rounded ; upper and lower 

 margins of left valve slightly convex, sub- 

 parallel ; right valve strongly convex, result- 

 ing in very marked increase in overlap near 

 mid-portion of the dorsum and venter ; over- 

 lap greatest just back of middle on dorsum, 

 just in front of middle on venter ; overlap 

 on ends inconspicuous; greatest thickness 

 just back of, and above center; both valves 

 have pinched or broadly depressed area in 

 front of point of greatest tumidity, greatest 

 at dorsum, disappearing about mid-height; 

 hinge straight, occupies anterior two-thirds 

 of dorsal articulation ; surface smooth. 



Length, 0.63 mm; height, 0.43 mm; 

 thickness, 0.26 mm. 



Vienna formation, locality 10, common. 



Genus Sargentina Coryell and Johnson 



Sargentina allani Coryell and Johnson 



Plate 6, figures 19-22 



Sargentina allani Coryell and Johnson, 1939, Jour. 



Paleontology, voL 13, p. 223, pi. 25, figs. 9a-c, 



Clore formation. 

 Sargentina forsetii Coryell and Johnson, idem. p. 



224, pi. 25, figs. 10a, b, Clore formation. 



Male: length 0.91 mm; height, 0.61 

 mm ; thickness, 0.39 mm. 



Female: length, 0.95 mm; height, 0.62 

 mm; thickness, 0.49 mm. 



Clore formation, locality 27, common. 



These two species are similar in every 

 respect except thickness. Because the greater 

 thickness of the posterior portion of the 

 shell has been considered a character that 

 distinguishes female individuals, these two 

 forms are believed to be the two sexes of 

 the same species. 



The figured specimens are topotypes. 

 figure 19 clearly showing the obesity of 



