142 SHELVE DISTRICT 



unequally parvicostellate with 7 ribs per mm, 10 mm antero-medially of the dorsal 

 umbo ; ventral muscle scar feebly impressed subcircular, as long as wide and 

 extending anteriorly for 28% of the length of the pedicle valve ; cardinalia delicately 

 developed, notothyrial platform vestigial except in late growth stages. 



Description. Piano- to gently concavo-convex, transversely semi-elliptical 

 Rafinesquina with cardinal angles normally orthogonal but also obtuse or acute 

 especially the latter in young shells ; pedicle valve evenly convex, 8% as deep as 

 long (range 7% to 10% for 6 valves), brachial valve flat or slightly concave, rarely 

 with a dorsally deflected margin, 79% as long as wide with a strongly convex trilobed 

 protegulum 07 mm long ; ventral interarea apsacline with a large supra-apical 

 foramen and a small pseudodeltidium, dorsal interarea narrow anacline with a large 

 convex, medially indented chilidium ; radial ornamentation unequally parvicostel- 

 late, rarely with a thickened median rib, with counts of 6, 7, 8 and 9 ribs per 

 mm, 10 mm antero-medially of the umbones of 10, 13, 3 and 1 brachial valves 

 respectively. 



Ellipsoidal teeth obliquely disposed parallel with the widely divergent dental 

 plates reaching anteriorly for 9% of the length of the pedicle valve ; ventral muscle 

 field subcircular, 99% as wide as long and extending forward for 28% of the length 

 of the valve, usually faintly developed and consisting of a broad adductor scar 

 impressed on either side of a median ridge and a pair of flanking semicircular 

 diductor scars ; internal surface rarely coarsely tuberculate, mantle canal system 

 unknown. 



Cardinal process lobes delicate, plate-like, slightly splayed and ankylosed to widely 

 divergent thin socket ridges extending anteriorly for 9% of the length of the brachial 

 valve and 31% of their lateral extension ; concave surfaces of narrow sockets 

 normally tuberculate in adult valves ; notothyrial platform poorly developed 

 except as a low anchor-shaped feature in late adult growth stages so that the car- 

 dinalia usually bound slight postero-medial hollows in younger valves ; median 

 ridge and transmuscle septa rarely developed, extending forward for about two-fifths 

 the length of the valve ; only the posterior pair of reniform adductor scars known, 

 extending anteriorly for 30% of the length of the valve ; subperipheral rim, up to 

 2 mm wide, rarely developed. 



Type material. 



Holotype External and internal moulds of brachial valve 



(BB 35558a, b) 

 Paratypes External and internal moulds of pedicle valve 



(BB 35559a, b) 



External and internal moulds of pedicle valve 



(BB 35560a, b) 



External and internal moulds of pedicle valve 



(BB 35561a, b) 



Incomplete internal mould of brachial valve 



(BB 35562) 



length 



width (mm 



12-5 



17-0 



13-0 



14-5 



14-0 



- 



n-5 



12-0 



