ORDOVICIAN AND SILUJUAN BRACHIOPODA OF THE GTRVAN DISTRICT. 853 



brachial valve, and fig. 18 is the internal cast of the same specimen; fig. 19 is a 

 restoration from an imperfect internal cast of a pedicle-valve, but the ribbing, as far 

 as can be judged from the margins, seems to be coarser and the ribs straight instead 

 of curved at the sides as in the figured brachial valve, so that the reference to the 

 same species appears doubtful. The amended and enlarged specific description 

 should therefore be as follows : — 



Shell transversely subelliptical to subquadrate ; hinge-line rather less than maxi- 

 mum width of shell ; cardinal angles obtuse to subrectangular. Brachial valve 

 moderately convex, with weak narrow median depression from beak to margin ; beak 

 very small, not swollen or projecting, inconspicuous ; hinge-area moderate, at right 

 angles to plane of valve. Interior of valve with thickened transverse cardinal plate ; 

 cardinal process thin, linear, straight, with a short groove on each side diverging 

 from it anteriorly ; crura very stout, triangular, prominent ; dental sockets small ; 

 low broad ridge separating adductor impressions ; posterior adductors small, deep, 

 well marked ; anterior adductors smaller, indistinctly denned ; pair of short median 

 closely placed parallel vascular sinuses running straight forwards from front of 

 median ridge ; two lateral pairs of straight parallel closely placed divergent vascular 

 sinuses, each lateral pair making an angle of about 30° with median line ; ovarian 

 areas covering space between cardinal line, and lateral sinuses marked with fine 

 broken irregular radiating lines and pits ; similar markings in middle between lateral 

 sinuses. 



Surface of valve ornamented with fine subangular closely placed ribs, curving 

 back on each side, subequal on margin and there numbering 120-140, increasing 

 in number firstly by intercalation close to beak, secondly at about half their length, 

 and often again with considerable regularity at about three-fourths their length, 

 rarely bifurcating. Interspaces subangular, subequal to or wider than ribs ; both 

 covered with close concentric striae, coarsest and most distinct near margin. 



Dimensions. — Length, 16*0 mm. ; width, 20*5 mm. 



Horizon. — Balclatchie Group. 



Localities. — Ardmillan, Dow Hill, Balclatchie. 



Remarks. — The above description is from the type-specimens (Davidson's 

 figs. 17 and 18), which are the external impression and internal cast of a brachial 

 valve ; but several pedicle-valves with well-marked characters may be referred to the 

 same species from their agreement in shape, size, ribbing, development of vascular 

 sinuses, and occurrence on the same horizon at the same locality. 



The description of them is as follows : — Pedicle-valve gently convex, transversely 

 semi-elliptical ; cardinal angles rectangular ; beak rather high, pointed, scarcely in- 

 curved, rising well above hinge-line ; hinge-area triangular, steeply inclined, larger 

 than that of opposite valve. Interior with pair of short, stout teeth ; diductors 

 short, small, elongated, triangular, confined to interior of beak, separated by narrow 

 adductor scar and prolonged anteriorly into pair of parallel broad well-marked 



