ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 841 



thin low laminar cardinal process and a broad subquadrate muscle-scar composed of 

 two pairs of deeply impressed adductors of which the anterior pair is the larger and 

 expands anteriorly ; a strong median ridge is present, and the genital areas are 

 pitted and ridged. 



As regards the external shape it may be mentioned that the pedicle-valve is 

 usually deeper and more conical, and has a larger hinge-area than the other valve, 

 but the median sinus is less marked or may be absent. The number of ribs varies, 

 and occasionally the median rib divides into 2 or 3 subequal ones a short distance 

 from the beak. 



This species is closely allied to 0. crispa, M'Coy, which occurs on a higher 

 horizon in the Girvan area, but the former differs in the shape of the muscle-scar 

 in the pedicle-valve, in the beaks of the valves being more equal in size, in the 

 brachial valve being more convex, in the teeth being larger and stouter, and in 

 the ribs being fewer in number. The shell is also usually smaller. The American 

 species 0. (Heb.) bellarugosa, Conrad,* from the Galena shales of Minnesota, bears 

 a considerable resemblance to it. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group. 



Localities. — (l) Craighead; (2) Balclatchie (shales and conglomerate), Ardmillan. 



Orthis (Hebertella) bellatrix, sp. nov. 



(Plate VII, figs. 21-28 ; Plate VIII, figs. 1, 2, ? 31.) 

 1883. Orthis sowerbyana, Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., pp. 178, 226. 



Shell unequal, biconvex, transversely subelliptical ; hinge-line straight, rather less 

 than width of shell ; cardinal angles obtuse or slightly rounded. Pedicle-valve less 

 convex than brachial valve, with very shallow broad median sinus or flattening of- 

 surfape causing wide weak sinuation of front margin ; beak moderately high, slightly 

 incurved, prominent, with high triangular hinge-area steeply inclined to plane of valve ; 

 delthyrium large. Interior with elongate subpentagonal to oval muscle-scar, about 

 one-fourth the length of the valve, deeply sunk and circumscribed, composed of a 

 pair of small elongate diductors with narrow linear adductor scar between them ; 

 teeth strong, short. 



Brachial valve swollen, more convex and deeper than opposite valve ; beak low, 

 small, inconspicuous, not projecting ; hinge-area narrow, nearly in plane of valve. 

 Interior with long narrow linear cardinal process, swollen at base ; strong short 

 crural plates ; adductor scars separated by low broad rounded ridge ; posterior 

 adductors large, circular, deep, well marked ; anterior adductors smaller, indistinct. 



Surface of valves ornamented by numerous subequal subangular ribs, slightly 



curved back on sides, increasing rather irregularly by division into 2 or 3 at 



about one-fourth their length, and frequently again at half or two-thirds their length, 



and rarely by intercalation ; ribs finer and closer together towards cardinal angles, 



* Winchell and Schuchert, Fal. Minnes., vol. iii, p. 434, pi xxxiii, figs. 1-4. 



