122 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



genus, most similar to that of C. bowsheri (Cooper), and is strongly 

 curved longitudinally, in contrast to the nearly straight camarophorium 

 of species of Atribonium. 



There is no trace of a stolidium on any of the numerous specimens 

 in the Easton (1962) collection, and the form of the anterior commis- 

 sure is typical of that of species in which the stolidium is absent. 



Comparisons. — Coledium obesum is characterized by its subglobular 

 shape, sharp, suberect beak, with open delthyrium and foramen that 

 penetrates the apex, its short but strongly curved camarophorium, and 

 especially by its few and gentle costation of fold and flanks. It most 

 nearly resembles C. bowsheri (Cooper) in general shape, differing in 

 its less transverse outline, and fewer weaker costae. It also resembles 

 C. explanatum (McChesney), but is somewhat smaller and its costae, 

 on both fold and flanks, are much weaker. Its maximum size is only 

 about half that of C. erugatum n. sp. or C. torvum n. sp. ; its costae 

 further distinguish it from the former, its fewer and weaker costae from 

 the latter. It is smaller, wider, and less triangular in outline than C. 

 rhomboidale (Hall and Clarke) and C. therum (Walcott), and its 

 costae are weaker. It is smaller, less convex, and has weaker costae on 

 the fold than C. cestriensis (Snider) . 



Remarks. — Shaw (1962, p. 633) described specimens from the 

 Lodgepole Limestone of Montana under the designation of Stenocisma 

 (sic) obesa (Clark). Study of his illustrated specimens in the U.S. 

 National Museum collection shows that they have the flattened ante- 

 rior, and the general form that typify species of Atribonium, and are 

 not closely related to Coledium obesum. Shaw's specimens probably 

 belong to a new species that is related to Atribonium bisinuatum 

 (Rowley). He suggested that his new species Stenocisma uniplicatum 

 Shaw probably is a variant of the one that he called 5. obesa, but that 

 it is stratigraphically segregated. This idea is corroborated by the fact 

 that the range of variation of the closely related A. bisinuatum (Row- 

 ley) includes specimens that resemble Shaw's two categories {see 

 Weller, 1914, pi. 23). However, variants of the Mississippi Valley 

 population may have become established as separate species in Mon- 

 tana, and as long as the two forms are stratigraphically segregated, I 

 agree with Shaw that they should be recognized as distinct. 



Growth. — Trends of growth in this species are typical for the genus, 

 with points on the width scattergram (fig. 29) clustered narrowly 

 along a straight line, and those on the thickness scattergram more 

 widely spread along a curve of gentle acceleration. Small shells are 



Fig. 29. — Scatter diagrams of dimensions of Coledium obesum (Clark) from 

 Easton's (1962) locality 13414, Golden Valley County, Mont. 



