98 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



would imply that these various forms diverge at the outset, that they 

 in other words represent a sudden explosion, and therefore represent 

 distinct specific groups." 



If the number of costae in the four species of the C. "tingi" group 

 is invariable, these are the only known species of stenoscismatacean in 

 which that is true. To suppose that if the biplicate form were con- 

 specific with the tricostate form, some specimens with two and one- 

 half costae must necessarily occur, is biologically naive. More likely, 

 the C. "tingi" group is a normal species, containing individuals with 

 two to five costae in the sulcus. Further evidence that they constitute 

 a single species: (1) their occurrence — they all occur together; (2) 

 their numbers — a qualitative normal distribution curve could be con- 

 structed from Grabau's statements of abundance. Concerning C. bitingi 

 he says, "Less common than the next" ; for C. tritingi it is, "This 

 species is the most common"; for C. quadritingi, "Occurs with the 

 preceding, but less common" ; and C. pentatingi is "very rare." The 

 first valid species in the group is C. bitingi (p. 88) ; the others belong 

 in its synonymy. 



I have not been able to examine specimens of C. bitingi (as emended 

 here), but Grabau's descriptions and illustrations (including a sketch, 

 Text-fig. 8, p. 87, of an apical cross-section) show specimens that seem 

 to fit the definition of Coledium. They definitely are stenoscismatacean, 

 with ventral spondylium and dorsal camarophorium. Their valves 

 meet at the anterior to form an obtuse angle, but not in a plane or 

 a gentle curve as in species of Atribonium. The valve edges protrude 

 slightly, as in some other species of Coledium (e.g., C. angarium n. sp. ; 

 C. therum (Walcott)) ; they have numerous costae on the flanks, but 

 these are very low and weak, similar to costation of C. undulatum n. 

 sp. Until it is possible to examine authenticated specimens of C. bitingi, 

 the evidence makes it reasonable to group them with other species of 

 Coledium. 



Range. — The earliest known species of Coledium occurs in the 

 Middle Devonian. The genus is most abundant in the Mississippian 

 and, although rare in the Pennsylvanian, it is nevertheless the most 

 common stenoscismatacean in that period. Its range terminates in the 

 Permian of Timor. 



COLEDIUM ERUGATUM Grant n. sp. 



Plate 10, figs. 2-3 ; figures 20, 21 



Shell average size for genus, moderately to strongly biconvex ; out- 

 line subpentagonal to subelliptical, length and width nearly equal; 

 commissure uniplicate with high fold at anterior, producing only short 

 and low fold along brachial valve; sulcus shallow, rather broadly 



