884 DR F. R. C. REED ON THE 



the length of valve. Brachial valve flat or very slightly concave ; beak incon- 

 spicuous ; interior with large subflabellate divergent muscle-scars about one-third to 

 one-half the length of valve, with thickened crinkled anterior margins, and each 

 muscle-scar divided by narrow radial ridge into anterior and posterior adductor. 

 Surface of valves ornamented with 12-16 unusually strong straight primary radii, 

 generally equidistant, with a shorter secondary one in each interspace and 3-5 finer 

 thread-like lines between. Interiors with coarse pustules radially arranged along 

 the lines. 



Dimensions. — -Length, 2'75-4"5 mm. ; width, 5"50-7'0 mm. 



Horizon. — Stinchar Limestone Group. 



Locality. — Craighead. 



Remarks. — This small shell is certainly distinct from Sowerby's type of PI. sericea. 

 and more resembles PI. quinquecostata, var. balclatchiensis. In external appearance 

 it resembles PI. sericea, var. albida, but the internal characters are distinctive. 



Plectambonites sericea (Sowerby), var. rhombica (M'Coy). 

 1852. Lepteena sericea, var. rhombica, M'Coy, Synop. Brit. Palxoz. Foss, Woodw. Mus., p. 239. 

 1870. Leptiena sericea, var. rhombica, M'Coy, Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 325, 



pi. xlviii, figs. 20-22. 

 1883. Leptsena sericea, Davidson (pars), Mon. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 227. 



From a comparison of M'Coy's types of PI. sericea, var. rhombica, with some 

 poorly preserved shells from Shalloch Mill it seems probable that this variety occurs 

 at this locality ; but the lateral angles are more pointed and acute than Davidso>: 

 showed in his figures of Welsh examples. The Shalloch Mill form differs from the 

 variety conspicua in the more swollen, more prominent, and incurved beak of the 

 pedicle-valve, and more uniform convexity. Primary, secondary, and tertiary radii 

 seem more numerous, and the muscle-scars of the brachial valve seem identical with 

 those of the PI. sericea from Llanfyllin, figured by Davidson, but no median septum 

 appears to be present. No cardinal denticulations have been observed. 



Dimensions. — Length, 18 '5 mm. ; width, 44 '0 mm. 



Horizon. — White house Group. 



Locality. — Shalloch Mill. 



Plectambonites sericea (Sowerby), var. nov. semirugata. 

 (Plate XV, figs. 18-26.) 

 1873. Leptxna quinquecostata, M'Coy, Salter, Cat. Camb. Silur. Foss. Woodw. Mus., p. 64. 

 Shell transversely semielliptical, plano-convex to concavo-convex, about twice 

 as wide as long ; cardinal angles acutely pointed at about 45°-60°, sometimes slightly 

 produced and subalate ; cardinal margin with 6-8 denticulations on each side of 

 beak, decreasing in size to lateral angles. Pedicle-valve gently convex, with lateral 

 portions somewhat flattened ; beak small, pointed, not projecting, scarcely incurved ; 

 interior with pair of small short pointed subrhomboidal diductors diverging at 



