878 DR F. R. C. REED ON THE 



equidistant radii diverging at 45° apart, forming a distinct median group ; interspaces 

 very delicately striated. Internal characters insufficiently known. 



Dimensions. — Length, 4 mm. ; width, 8 mm. 



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



Localities. — (l) Balclatchie, Ardmillan, Dow Hill ; (2) Craighead. 



Remarks. — This variety is distinguished from the type of the species by its shape, 

 less swollen beak, and the marked group of 3 radii dividing the surface into 4 sub- 

 equal areas ; one of these radii is always central. 



Plectambonites quinquecostata (M'Coy), var. nov. cylindrica. 

 (Plate XIV, figs. 11-17.) 



1870. Leptsena quinquecostata, M'Coy, Davidson (pars), Mon. Brit. Foss. Brack., vol. iii, pt. vii, 



p. 332, pi. xlviii, fig. 23 (non figs. 24-27). 

 1883. Leptxna quinquecostata, M'Coy, Davidson (pars), ibid., vol. v, Silur. Suppl., pp. 169, 227. 



Shell transverse, subfusiform, subtriangular, with pointed lateral angles. Pedicle- 

 valve subtriangular, strongly inflated to subcarinate in middle ; lateral portion:; 

 depressed ; cardinal angles more or less produced, subcylindrical, acutely pointed : 

 beak swollen, strongly incurved. Brachial valve strongly concave. Surface of 

 valves with 8-12 primary radii, usually weakly differentiated in strength from the 

 intermediate lines, with frequently a median group of 3-4 primaries strongly 

 developed and shorter secondaries intercalated. 



Dimensions. — Length, 7 mm. ; width, 12 mm. 



Horizon. — Whitehouse Group. 



Locality. — Shalloch Mill. 



Remarks. — The shape of the shell somewhat approaches PI. Etheridgei and 

 PI. transversalis, var. Duvali (see p. 887), and resembles the original Irish type of 

 PI. quinquecostata. The internal characters are unknown. 



Plectambonites quinquecostata (M'Coy), var. nov. depressa. 

 (Plate XIV, figs. 18,19.) 



1883. Leptsena quinquecostata, M'Coy, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 pp. 169, 227. 



Shell gently plano-convex, compressed, semicircular to transversely semielliptical ; 

 cardinal angles subrectangular or obtuse, flattened. Pedicle-valve very gently 

 convex ; beak not swollen, nor incurved, nor overhanging. Brachial valve flat or 

 very slightly concave. Surface with 3 main primary radii diverging apart at 

 45°-60°, with usually 1 or 2 shorter smaller secondary or tertiary ones intercalal 

 and occasionally a few small concentric rugae near beak. Interior unknown. 



Dimensions. — Length, 9 mm. ; width, 18 mm. 



Horizon. — Whitehouse Group. 



Localities. — Shalloch Mill, Whitehouse Bay. 



