Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Table 24. — Measurements of Coledium explanatum from the Chester 2 miles 

 north of Baldwin, Randolph County, III. 



Measurements, in millimeters 



foration, and especially by its strong costae that number nearly in- 

 variably two on the fold. It is about the same size as C. obesum 

 (Clark) and C. bowsheri (Cooper), but is more strongly costate than 

 either. It is much smaller than C. erugatum n. sp. and C. torvum n. sp., 

 and also more strongly costate than either. McChesney (1860) com- 

 pared it to Rhynchonella wortheni Hall, which, according to Weller 

 (1914), lacks the internal structures of a stenoscismatacean. Its two 

 costae on the fold recall C. rhomboideum (Phillips) but that species 

 is more pentagonal, less convex, and lacks costae on the flanks (see 

 pi. 18, fig. 1). The rather strong anterior costae of C. explanatum 

 resemble those of C. pleurodon (Phillips) (pi. 18, fig. 3) and C. nu- 

 culum (Schellwien) (pi. 18, fig. 4), but the number of costae in C. 

 explanatum typically is fewer than in either of those species, and the 

 convexity of the shell much lower than in C. nuculum. 



Discussion. — According to Weller (1914), and judging from the 

 USNM collection, this species is the most abundant and widespread 

 species of Coledium. The form of the holotype is known from casts, 

 one of which formed the basis for Hall and Clarke's illustrations (see 

 synonymy). This specimen is somewhat more strongly costate than 

 most specimens in the USNM collections, and more so than specimens 

 illustrated by Weller (1914, pi. 23) from the Chester of Illinois, the 

 topotype area. However, agreement of the holotype with abundant 

 specimens from the same region is close in all other features, and 

 Weller's use of the name "explanatum" is here followed. 



Occurrence and abundance. — Upper part of Chester Series, just east 

 of Marion-Fredonia road, 2 miles south of Marion, Ky. (71 specimens 



