124 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



nearly as wide as they are long, but larger ones are more transverse. 

 Only a few specimens are thicker than long, although the trend is 

 toward increasingly equal length and thickness. Projection of the 

 lower ends of the curves would continue the same trends, indicating 

 little or no allometric growth in early postlarval stages. 



Table 25. — Measurements of Coledium obesum from the Heath Shale locality 

 13414 of Easton (1962), Mont. 



Measurements, in millimeters 



Occurrence and abundance. — Heath Shale, Montana, Easton local- 

 ity No. 13414 (about 65 specimens). 

 Age. — Late Mississippian. 



COLEDIUM OPIMUM Grant n. sp. 



Plate 18, figs. 5-5a 



Shell about average size for genus, juveniles flatly biconvex, adults 

 strongly biconvex ; outline rounded subpentagonal, widest near mid- 

 length, slightly longer than wide ; adults uniplicate, fold moderately 

 high, broad at anterior margin, but standing only slightly above 

 flanks, beginning about 5 mm. anterior to brachial beak ; sulcus shal- 

 low, with flattened trough ; juveniles shorter than 5 mm. with anterior 

 margin flattened or slightly emarginate on midline, anticipating fold ; 

 costae very low, rounded, numbering two on fold, one in sulcus, none 

 on flanks ; posterolateral edges of brachial valve strongly overlapping 

 pedicle valve ; valves meeting at obtuse angle around anterior margin, 

 slightly protruding at extreme edges, but without trace of stolidium ; 

 growth laminae weak, widely spaced. 



Pedicle valve somewhat inflated, especially in umbonal region ; beak 

 thick, without beak ridges, strongly curved, pressed against brachial 



