828 DR F. R. C. REED ON THE 



slightly convex, with the beak forming the apex and rising high above the hinge- 

 line, but not much incurved ; the hinge-area is large and steeply inclined, and has 

 a large triangular delthyrium. The muscle-scar in this valve is very short, being 

 limited almost entirely to the beak, and it is deeply sunk and well circumscribed. 

 .The brachial valve is flattened, but has occasional traces of a weak median depression. 

 There are 24-30 simple equal regular straight rounded ribs, separated by subequal 

 or slightly narrower rounded interspaces, and both the ribs and interspaces are 

 ornamented by fine longitudinal lines, while the interspaces usually show strong 

 concentric striae. 



Horizon. — Stinchar Limestone Group. 



Localities. — Craighead, Bogang. 



Orthis calligramma, Dalman, var. nov. subplicata. 



(Plate V, figs. 10-15.) 



1883. Orthis calligramma, Dalman, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 



p. 225. 

 1883. Orthis rustica, Sowerby, Davidson (pars), ibid., p. 226. 



Shell plano-convex, subquadrate, rounded to semielliptical ; hinge-line rather less 

 than width of shell or equal to it ; cardinal angles obtuse or subrectangular. Pedicle- 

 valve strongly convex, swollen ; beak rather high, incurved ; hinge-area large, 

 triangular, lying in plane of valve or steeply inclined. Brachial valve flattened, with 

 very weak median depression widening anteriorly ; beak small, inconspicuous, not 

 raised ; hinge-area narrow, nearly at right angles to valve. 



Interior of pedicle-valve with broad subtruncate short triangular muscle-scar, 

 rather deeply impressed, less clearly defined in front, composed of two narrow 

 diductors enclosing adductors of about one-third the width of the scar ; a pair of 

 short broad rapidly disappearing subparallel vascular sinuses run forward from 

 diductors ; teeth short, small. 



Interior of brachial valve with thickened cardinal plate, long narrow linear cardinal 

 process, and very short stout triangular crural plates ; muscle-scars obscure. 



Surface of valves ornamented with 20-26 rounded ribs of equal strength and 

 equidistant on margin ; about half the number are primaries, the others being inter- 

 calated in the interspaces or branching off one side of the primaries at about one- 

 fourth to one-half their length. Interspaces wider than ribs, rounded, with strong- 

 concentric striation. Ribs curve back slightly on both sides towards cardinal angles. 



Dimensions. — Length, 20 mm. ; width, 24 mm. 



Horizon. — Mulloch Hill Group. 



Locality. — Mulloch Hill Wood. 



Remarks. — These shells were mostly labelled by Davidson O. rustica, but they 

 are quite distinct in their convexity and ribbing. The interior of the brachial valve 



