96 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Pedicle valve interior with dental plates converging to form deep, 

 boat-shaped spondylium, commonly meeting above floor of valve to 

 fuse and form low median septum duplex, elevating spondylium above 

 floor, but intersecting floor in apex of valve (within 2 mm. of beak) 

 in some species to produce apically sessile, anteriorly elevated spon- 

 dylium ; median septum extending farther anterior than spondylium, 

 normally slightly less than half length of valve ; muscle and pallial 

 markings not observed. 



Brachial valve interior with short hinge plate ; camarophorium be- 

 ginning in apex, expanding anteriorly as shallow trough on increas- 

 ingly high median septum, extending farther to anterior than septum, 

 somewhat beyond midlength of valve and shell; septum extending 

 about one-third length of valve ; posterior part of trough of camaro- 

 phorium united to underside of hinge plate by low intercamarophorial 

 plate ; apex of hinge plate swollen to form knoblike, finely fimbriate 

 cardinal process ; crura extending from anterior edge of hinge plate, 

 curving ventrally above trough of camarophorium, conforming to its 

 curvature, not observed to extend as far forward as camarophorium ; 

 muscle and pallial markings not observed. 



Type species. — Coledium erugatum n. sp. 



Comparisons. — Coledium is characterized by its small size (for a 

 stenoscismatid), slight overlap of the posterolateral margins of the 

 valves, angular meeting of the anterior margins of the valves, with 

 slight outpushing of the valve edges in some species and development 

 of a narrow stolidium in others, longitudinally strongly curved cam- 

 arophorium, and by its normally weak costation, especially on the 

 flanks. It differs from Atribonium in its higher fold and deeper sulcus, 

 normally weaker costation, angular meeting of the anterior edges of 

 the valves, overlapping of the posterior edges, strongly curved rather 

 than nearly flat camarophorium, and in presence in some species of a 

 narrow stolidium. 



Coledium probably is ancestral to Stenoscisma; it resembles that 

 genus by possession of a stolidium. It differs in the inconsistent pres- 

 ence of the stolidium, the narrowness of the stolidium where it is 

 present, the normally narrower outline of the shell, and weak or ab- 

 sent costae. Some Permian species of Stenoscisma are abnormally 

 smooth for that genus, but none is as smooth as normal in Coledium; 

 furthermore, the Permian species are wider, more triangular (with 

 greatest width farther anterior), and normally possess stronger costae 

 on the flanks. 



Coledium probably descended from Atribonium, or both genera de- 

 rived from a common ancestor in the Late Silurian or Early Devonian. 

 The earliest known species of Coledium is C. rhomboidale (Hall and 



