48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



ATRIBONIUM GREGERI (Branson) 



Plate 3, figs. 1-1E; figures 8, 9 



Hypothyris gregeri BRANSON, E. B., 1923, Missouri Bur. Mines & Geol., vol. 



17, 2d sen, p. 91, pi. 17, figs. 5-7, 11-14. 

 Camerophoria gregeri (Branson) STAINBROOK, M. A., 1942, Journ. Paleont., 



vol. 16, No. 5, p. 615, pi. 88, figs. 16-21 ; text fig. 6. 



Supplementary description. — Shell large for genus, strongly bicon- 

 vex ; globose ; outline subcircular to subelliptical, width less than length 

 in small specimens, more than length in large specimens, greatest width 

 slightly anterior to midlength ; commissure uniplicate, with broad, high, 

 flat-crested fold at anterior, expressed only as slight elevation of me- 

 dian part of valve ; sulcus shallow, scarcely depressed below flanks, 

 broad, flat, extending anteriorly as broad tongue into deep fold ; costae 

 low, rounded, most simple, some bifurcating, beginning about 5 mm. 

 anterior to beaks, numbering four to seven on fold, one fewer in 

 sulcus, three to five on each flank ; anterior margin of small shells flat- 

 tened, of larger shells evenly convex ; growth lines fine, closely and 

 regularly spaced; growth laminae only slightly stronger, irregularly 

 and more widely spaced. 



Pedicle valve moderately convex transversely, more strongly convex 

 longitudinally through sulcus ; beak short, sharp, nearly straight ; beak 

 ridges distinct but blunt ; posterolateral slopes slightly flattened, pinch- 

 ing in toward commissure in some specimens ; delthyrium small tri- 

 angular, nearly filled by small, triangular, conjunct deltidial plates, 

 leaving minute foramen. 



Brachial valve more strongly convex in both directions, slightly 

 flattened along crest of fold ; beak rather sharp, apex in pedicle valve 

 hidden by deltidial plates. 



Pedicle valve interior with dental plates converging, meeting one 

 another to form spondylium ; spondylium attached to floor of valve in 

 apex, becoming elevated anteriorly on low median septum duplex ; 

 spondylium extending anteriorly about one-fourth length of valve, 

 septum extending slightly farther. 



Brachial valve interior with camarophorium supported by high 

 median septum, extending anteriorly about half length of shell, beyond 

 midlength of brachial valve ; camarophorium beginning against floor 

 of valve slightly anterior to apex of beak, joined to hinge plate by low 

 intercamarophorial plate ; hinge plate undivided, with slight swelling 

 at posterior forming cardinal process ; crura extending anteriorly from 

 edge of hinge plate, above camarophorium, extending slightly farther 

 anterior than camarophorium. 



Lectotype (here designated) . — Hypothyris gregeri Branson (1923, 

 pi. 17, figs. 11-14) Callaway Limestone, Callaway County, Mo. 



