102 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



of U.S. Highway 62, 7.5 miles east of Fort Gibson, Okla. (1 speci- 

 men 142132). 



Age. — Late Mississippian (low in Meramec). 



COLEDIUM ANGARIUM Grant n. sp. 

 Plate 12, figs. 2-2b; figure 22 



Shell small for genus, moderately to strongly biconvex; outline 

 subtrigonal to subelliptical, slightly elongate to strongly transverse; 

 fold low, broad, gently arched at anterior, standing only silghtly 

 above flanks; sulcus similarly shallow; costae low, rounded, begin- 

 ning about 5-7 mm. anterior to beaks (many shells without costae ; 

 average length being about 5-7 mm.), numbering up to four on fold, 

 one fewer in sulcus, normally none on flanks; valve edges slightly 

 protruding at anterior, especially on fold, producing rudiment of 

 stolidium: not continuous for more than 2 mm. along commissure; 

 growth lines very weak, crowded; growth laminae slightly stronger, 

 widely spaced. 



Pedicle valve flatly to moderately strongly convex, greatest swell- 

 ing in umbonal region; beak unusually long, nearly straight to sub- 

 erect, sharp, with well-defined beak ridges ; broad and long flattened 

 area between beak ridges and valve edges ; delthyrium small, trigonal, 

 constricted by pair of small deltidial plates (conjunction not observed), 

 leaving small, slit-shaped foramen, not penetrating apex of beak. 



Brachial valve more strongly convex, with greatest longitudinal 

 curvature in umbonal region ; beak curved into pedicle valve, apex 

 hidden by deltidial plates. 



Pedicle valve interior with dental plates meeting just above floor, 

 forming large, boat-shaped spondylium, sessile or nearly so at apex, 

 elevated anteriorly on low median septum duplex ; septum extending 

 anteriorly about one-third length of valve. 



Brachial valve interior with low cardinal process near apex, at pos- 

 terior of hinge plate; crura extending from anterior edge of hinge 

 plate; camarophorium connected to underside of hinge plate by thin 

 intercamarophorial plate, supported on high median septum duplex; 

 septum extending anteriorly about one-fourth length of valve, cam- 

 arophorium extending farther anterior, curving ventrally. 



Holotype.— USNM 142085, plate 12, fig. 2b. 



Comparisons. — Coledium angarium is characterized by its small 

 size, few or absent costae confined to the fold and sulcus, long sharp 

 pedicle beak with distinct beak ridges, and short, sporadic rudiment 

 of a stolidium. C. therum (Walcott) also has a rudimentary stolidium 

 on some specimens, but is much larger, little resembling C. angarium. 



