112 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



COLEDIUM EVEXUM Grant n. sp. 



Plate 15, figs. 1-10; figure 26 



Shell slightly below median size for genus, moderately to strongly 

 biconvex; outline transversely subpentagonal, greatest width near 

 midlength; commissure strongly uniplicate; fold high at anterior, 

 rounded or somewhat flattened, beginning about 4 mm. anterior to 

 brachial beak, not standing high above flanks posterior to margin; 

 sulcus shallow, normally slightly raised along midline, reducing amount 

 of depression below flanks, extending anteriorly as broad tongue into 

 fold, beginning about 7 mm. anterior to pedicle beak; costae low, 

 rounded, beginning far forward, absent from some specimens, number- 

 ing two or three on fold, one fewer in sulcus, normally absent from 

 flanks but some specimens with one or two on each flank ; valve edges 

 slightly overlapping along narrow flanges on posterolateral slopes, 

 butting elsewhere, slightly protruding around anterior margins of 

 some specimens; growth lines fine, distinct, closely spaced; growth 

 laminae stronger, widely and irregularly spaced. 



Pedicle valve rather strongly convex for genus, greatest swelling 

 anterior to umbonal region ; beak long for genus, weakly attenuate, 

 erect to slightly incurved, apex sharp, not pressed against brachial 

 umbo ; delthyrium small, triangular ; deltidial plates not observed, 

 probably absent ; foramen coincident with delthyrium, not penetrating 

 apex of beak. 



Brachial valve similarly convex but with greatest swelling farther 

 anterior ; beak obtusely pointed, apex within pedicle valve. 



Pedicle valve interior with dental plates forming boat-shaped spon- 

 dylium, sessile only for about 0.5 mm. in apex, from there elevated on 

 low median septum duplex, extending anteriorly about one-fourth 

 length of valve. 



Brachial valve interior with thin flat hinge plate in apex ; camaro- 

 phorium elevated on high median septum duplex, longitudinal curva- 

 ture moderate, extending anteriorly about one-fourth total length of 

 shell, trough of camarophorium relatively narrow throughout length ; 

 intercamarophorial plate connecting trough of camarophorium to un- 

 derside of hinge plate, extending abnormally far forward, nearly en- 

 tire length of camarophorium ; crura not observed. 



Holotype.— USNM 142147, plate 15, fig. 9. 



Comparisons. — Coledium evexum is characterized by its somewhat 

 more than normally swollen pedicle valve, transversely subpentagonal 

 outline, very weak costae that commonly are absent from the flanks 

 and are entirely absent from some specimens, slightly protruding ante- 

 rior valve edges that presage an incipient stolidium, and its short 



