62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



ATRIBONIUM SUCCIDUUM Grant n. sp. 



Plate 2, figs. 2-2b 



Shell small for genus ; outline subtrigonal to subpentagonal, widest 

 anterior to midlength; profile moderately biconvex to wedge-shaped, 

 greatest thickness at anterior ; commissure uniplicate, with moderately 

 high fold extending dorsally about two-thirds height of anterior sur- 

 face ; fold elevating brachial valve only at anterior, normally more than 

 5 mm. anterior to brachial beak ; sulcus shallow, beginning about 5 mm. 

 anterior to pedicle beak ; costae low, rounded, beginning about 3 or 4 

 mm. anterior to beaks, numbering three or four, rarely five on fold, one 

 fewer in sulcus, two or three on each flank anterior margin flattened, 

 gently convex to gently concave; growth lines fine, weak, closely 

 spaced ; growth laminae stronger, irregularly and widely spaced. 



Pedicle valve flatly convex, with greatest convexity just anterior to 

 beak, strong geniculation at anterior, beak long, sharp, suberect ; beak 

 ridges blunt but distinct ; delthyrium small, constricted by pair of con- 

 junct deltidial plates ; pedicle foramen elongate, oval, normally pene- 

 trating apex of beak. 



Brachial valve more strongly convex transversely, flatly convex 

 longitudinally, strongly geniculate at anterior; apex of beak beneath 

 deltidial plates in pedicle valve. 



Pedicle valve interior with spondylium sessile in posterior, elevated 

 on low median septum toward anterior, septum extending slightly 

 anterior to midlength of valve. 



Brachial valve interior with camarophorium beginning against valve 

 floor in apex, separated from hinge plate by intercamarophorial plate ; 

 median septum extending forward nearly three-fourths length of valve. 



Holotype.—USNM 141996, plate 2, fig. 2a. 



Comparisons. — Atribonium succiduum is characterized by its small 

 size, few and weak costae, conjunct deltidial plates, and proportionately 

 long median septum in each valve. It most nearly resembles A. rostra- 

 turn n. sp. from the Thunder Bay Limestone in its relatively long pedi- 

 cle beak, but differs in its smaller size, less trigonal outline, weaker and 

 fewer costae, and concave anterior surface in some specimens. It is 

 smaller and has fewer and weaker costae than A. simatum n. sp. or A. 

 cooperorum n. sp. ; it is larger and proportionately not as thick at the 

 anterior as A. kernahani (Whiteaves) ; smaller and much narrower 

 than A. savagei (Cooper) or A. illinoisensis (Cooper). In addition, A. 

 succiduum is so much smaller and less globose than A. gregeri (Bran- 

 son) and less strongly folded than A. pauperum (Belanski) that de- 

 tailed comparisons are unnecessary. It has more numerous costae and 

 is much thicker at the anterior than A. halli (Fagerstrom). 



