38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Pedicle valve flatly convex transversely, strongly flexed through 

 sulcus, slightly swollen in umbonal region; beak curved, somewhat 

 flattened, acute, with beak ridges distinct although normally short and 

 somewhat blunt; delthyrium small, triangular, sides normally con- 

 stricted by pair of small, triangular, conjunct or nearly conjunct del- 

 tidial plates ; foramen small, slit like. 



Brachial valve somewhat more convex transversely, strongly curved 

 near anterior margins to produce flattened anterior and flanks; um- 

 bonal region transversely convex, longitudinally slightly convex, rather 

 flat, or slightly depressed in some species ; brachial beak curved, nor- 

 mally within pedicle valve behind deltidial plates. 



Pedicle valve interior with strong dental plates converging slightly 

 toward floor, intersecting floor in apical region of some species, nor- 

 mally abruptly curved to meet one another above floor, fusing to form 

 short median septum duplex ; height of septum increasing only slightly 

 toward anterior; dental plates terminating near back, leaving fused 

 portion supported by median septum to form shallow spondylium ex- 

 tending forward about one-third valve length ; septum continuing for- 

 ward to near midlength. 



Brachial valve interior with short undivided hinge plate projecting 

 forward slightly along midline ; crura slender, extending forward from 

 anterior edge of hinge plate, somewhat tapered anteriorly, continuing 

 forward about one-fourth length of valve ; median septum beginning at 

 apex, supporting hinge plate; camarophorium beginning in apex as 

 slight swelling of septum about midway between floor and underside of 

 hinge plate, swelling and becoming wider anteriorly, each side ex- 

 panding and curving ventrally to produce spoon-shaped camarophor- 

 ium bisected by intercamarophorial plate supporting hinge plate ; inter- 

 camarophorial plate thinning and declining anterior to hinge plate, 

 disappearing about 2 mm. anterior to beak; camarophorium capping 

 median septum anterior to termination of intercamarophorial plate, 

 height of septum increasing anteriorly causing camarophorium to curve 

 ventrally, sides of camarophorium becoming thinner and more nearly 

 parallel to valve floor toward anterior, narrowing near termination 

 just short of midlength of shell, about one-third to one-half length of 

 brachial valve. 



Type species. — Atribonium simatum n. sp. 



Diagnosis. — Atribonium is characterized by its small size, wedge- 

 shaped to elliptical outline, flat to strongly convex profile with rounded, 

 flattened, or slightly emarginate anterior, high flat-crested fold at the 

 commissure that does not produce a high crest on the valve, short 

 mesial costae that begin far anterior to the beaks, well-developed lat- 

 eral costae, and lack of a stolidium. Internally the spondylium is com- 



