44 



CHESTER OSTRACODES OF ILLINOIS 



just back of posterior node; hinge depressed, 

 notched at ends; surface reticulate. 



Length, 0.74 mm; height, 0.45 mm; 

 thickness, 0.33 mm. 



Paint Creek formation, locality 2, rare; 

 Golconda formation, locality 28, depth 

 1134-74 feet, rare. 



Glyptopleurina vetula Cooper, n. sp. 



Plate 8, figures 44-46 



Carapace large, elongate, dorsum 

 straight, ends curved, venter complex; re- 

 tral swing fairly prominent ; medium sinus 

 deep, anterior one shorter, more shallow; 

 lobe connected to anterior inflation by low 

 curved costa subparallel to another which 

 curves around front of lobe; overlap prom- 

 inent around free margins ; hinge channeled, 

 overlapped at ends ; surface granulose ; 

 frill, if ever present, completely broken 

 away ; greatest thickness near anterior end ; 

 greatest height through lobe. 



Length, 0.30 mm; height, 0.49 mm; 

 thickness, 0.48 mm. 



Golconda formation, locality 19, rare. 



This species, like others from the Chester, 

 lacks the posterior lobe possessed by many 

 congeneric forms. It is probable that the 

 great inflation of the posterior end has pre- 

 cluded the development of this node, sug- 

 gesting that these forms are females. 



Genus Mesoglypha Cooper, n. gen. 



Shell large, ovate in lateral outline, 

 strongly convex, right valve much larger 

 than left, overlapping along free margins, 

 extending above dorsal margin ; surface 

 sculpture centered about small but deep pit 

 just in front of and slightly above center of 

 shell, consists of rather indistinct, broad, 

 subhorizontal costae covering about fifty 

 per cent of shell surface; hinge line straight, 

 slightly channeled, terminated at each end 

 by flap overlapping the left valve; hinge- 

 ment cardine, free margins frilled. 



Genotype. — AL mediocre n. sp.. Glen 

 Dean formation. 



Mesoglypha is similar to Glyptopleura 

 Girty but lacks the prominent ridges mark- 

 ing the entire shell surface, is less quadrate 

 in lateral outline. No species of Glypto- 

 pleura are known to be frilled. Sargentina 



does not possess the same hinge structure, 

 nor a pit (it has a sulcus) and it lacks the 

 transverse costae. 



Mesoglypha mediocre Cooper, n. sp. 

 Plate 8, figures 47-49 



Carapace tumid, ovate in lateral outline, 

 ends rounded ; dorsum and venter convex ; 

 right valve much larger than left, overlap- 

 ping around entire free margins, rising above 

 hinge on dorsal margin ; postero-dorsal 

 angle truncated ; forward swing moderate ; 

 hinge straight, cardine, terminated at each 

 end by large flap overlaping left valve at 

 cardinal angles; surface smooth, except 

 around pit, which area is marked by low, 

 broad, somewhat indistinct subhorizontal 

 costae ; pit just in front of center, quite deep 

 although of small diameter ; greatest thick- 

 ness in posterior quarter ; greatest height in 

 anterior quarter; remnant of a frill appar- 

 ently present at antero-ventral corner. 



Length, 0.93 mm; height, 0.59 mm; 

 thickness, 0.49 mm. 



Glen Dean formation, locality 11, 

 common. 



Genus Venula Cooper, n. gen. 



Glyptopleuridae with faint anastomosing 

 costae, subquadrate outline, characteristic 

 Glyptopleura hingement ; right valve larger, 

 overlapping around free margin ; pit small, 

 circular, almost midway between ends and 

 slightly above midheight. 



Genotype. — Primitiopsisf striatus Cro- 

 neis and Funkhouser. 



Venula has somew^hat the same outline as 

 Primitiopsis (Jones) but lacks the reticulate 

 surface, ventral swelling, the strong con- 

 vexity, and the terminal flanges of the older 

 genus. 



Venula striata 



(Croneis and Funkhouser) 



Plate 9, figures 26-27 



Prunitiopsis? striatus Croneis and Funkhouser, 

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

 vol. 33, p. 340, pi. 10, fig. 13, Clore formation. 



Carapace subquadrate, dorsum and ven- 

 ter straight, parallel; anterior end rounded, 

 posterior almost straight and vertical, giving 

 forward swing; in dorsal view distinctly 

 wedge-shaped, sides straight or very slightly 

 convex ; hinge line straight, somewhat san- 



