864 OR F. R, C. REED ON THE 



their internal characters are unknown, and it is possible that they should be com- 

 pletely removed to another genus. 



Rafinesquina concentrica (Portlock). 

 (Plate XI, figs. 13-16.) 



1843 Orthis expansa, Sowerby, var. concentrica, Portlock, Geol. Rep. Londond., p. 452, pi. xxxvii, 



fig. I- 

 1870. Strophomena expansa (Sowerby), Davidson (pars), Man. Brit. Foss. Brack., vol. iii, pt. vii, 

 pi. xlv, fig. 9 (hon cet.). 



Shell subquadrate to transversely semielliptical, wider than long ; hinge-line 

 straight, equal to maximum width of shell or slightly less than it ; cardinal angles 

 rectangular to subacute, rarely subalate. Pedicle-valve gently convex ; beak obtuse, 

 small, rising a little above hinge-line, slightly incurved ; hinge-area narrow, nearly 

 at right angles to plane of valve, striated along its length ; interior of valve with pair 

 of short strong large low teeth inclined at about 30° to hinge-line, rather far apart, 

 with inner face crenulated ; diductor muscle-scars large, broad, flabelliform, radial ly 

 rigid, extending nearly half the length of valve, with short narrow deep median 

 groove between them at their base in beak ; adductors forming narrow oval median 

 scars about half length of diductors and completely enclosed by them ; low rounded 

 strong continuous submarginal concentric thickening of shell. Brachial valve nearly 

 flat or slightly concave ; hinge-area very narrow ; beak small, inconspicuous, not 

 raised ; interior with submarginal concentric thickening ; cardinal process large, 

 bilobed, composed of pair of closely placed triangular sessile lobes ; muscle-scars well 

 marked, composed of subflabelliform posterior adductors, about one-fifth length of 

 valve, rather deeply sunk, and elongate narrow anterior adductors of twice the 

 length, indistinctly defined in front ; low rounded median longitudinal ridge between 

 adductors, and traces of radiating vascular sinuses in anterior lateral portions of 

 valve. Small strongly pitted transverse circumscribed area on each side of beak 

 adjacent to hinge-line. 



Interior of both valves with a few coarse low rounded concentric growth-ridges. 



Surface of valves covered with very numerous fine rounded thread-like primary 

 radii of equal strength, some continuous from beak to margin, others originating at 

 half or less than half their length ; interspaces with 1-3 much finer thread-like lines ; 

 several more or less developed continuous low concentric corrugations on surface, and 

 frequently 3-4 small short rugse on each side of beak. 



Dimensions. — 



Length 



Width 

 Horizon. — Balclatchie Group. 

 Localities. — Dow Hill, Ardmillan. 



(1) 



(2) 



(3) 



18 



31 



30 mm 



28 



42 



35 mm 



