104 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



potential in the stock, and probably indicates descent of Stenoscisma 

 from a species of Coledium. Possibly the slight protrusion of the valve 

 edges functioned in directing the flow of currents in and out of the 

 shell in these species of Coledium, realizing its great development as 

 a stolidium only later, in the Permian, when adult individuals of 

 Stenoscisma adopted the habit of living free on the sea floor. The long 

 beak and relatively large pedicle foramen of C. angarium probably 

 mean that the species lived attached throughout its life, the rudimen- 

 tary stolidium having no role in adaptation to a free-living habit at this 

 early stage in the history of the family. 



Occurrence and abundance. — Barnett Formation, calcarenite 107 

 feet above base, at Barton Ranch, hill south of Bee Branch, 3,200 feet 

 S. 14° W. of Whites Crossing, 10 miles southwest of Mason, Tex. 

 (6 specimens USNM 142089) ; Barnett Formation, calcarenite, at 

 Barton Ranch 1,600 feet N. 88° W. of Whites Crossing, 10 miles 

 southwest of Mason, Tex. (9 specimens USNM 142088). 



Age. — Late Mississippian (upper Meramec or lower Chester) . 



COLEDIUM BOWSHERI (Cooper) 



Plate 9, figs. 2-2c 

 Stenoscisma bowsheri COOPER, G. A., 1956a, Journ. Paleont., vol. 30, No. 3, p. 

 525, pi. 61D, figs. 42-55. 



Original description (Cooper, 1956a). — Small, wider than long, 

 subpentagonal in outline ; greatest width at the middle ; sides narrowly 

 rounded; anterior margin nearly straight. Anterior commissure 

 strongly uniplicate. Surface smooth except for the fold and sulcus 

 which are occupied by three and two costae respectively. 



Pedicle valve gently convex in lateral profile and with the maximum 

 convexity just posterior to the middle; anterior profile broadly and 

 gently convex. Beak small, incurved. Umbo narrowly rounded, and 

 median region gently inflated, flanks moderately steep. Sulcus orig- 

 inating just anterior to the middle, shallow and bounded on each side 

 by a short oblique and angular fold. Tongue long and truncated ante- 

 riorly. 



Foramen triangular, open, no deltidial plates having been seen in 

 any of the specimens studied. 



Brachial valve deeper than the pedicle valve but with a gently con- 

 vex lateral profile ; anterior profile strongly arched. Fold low, orig- 

 inating at about the middle ; flanks gently swollen and precipitous. 



Pedicle valve interior with a shallow and short spondylium ; interior 

 of brachial valve having a strongly elevated but shallow torynidium 

 (camarophorium) anteriorly trilobed, the median lobe projecting 

 noticeably beyond the lateral ones. 



