882 DR F. R. C. REED ON THE 



Plectambonites sericea (Sowerby). 

 1839. Leptxna sericea, Kowerhy, in Murchison's Silurian System, p. 636, pi. xix, figs. 1 (1 2). 



The name Plectambonites sericea has been applied by most authors indiscriminately 

 to shells of a certain type from various Ordovician horizons, though they show con- 

 siderable differences. The stratigraphical or zonal separation of these forms requires 

 much further detailed work, but probably definite varieties or species will be found to 

 mark different horizons. In the case of the Girvan specimens an attempt is here made 

 to split up the heterogeneous assemblage of forms usually included under the one 

 specific name, but at present varietal names are only applied. 



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

 (Plate XIV, figs. 42-46 ; Plate XV, figs. 1-3.) 



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

 pp. 169, 227. 



Shell small, semielliptical, gently concavo-convex to plano-convex, compressed ; 

 cardinal angles rectangular. Pedicle-valve gently convex ; beak small, not swollen 

 nor prominent and scarcely incurved. Brachial valve nearly flat. Surface of shell 

 ornamented with fine subequal radial lines, with usually about 8-12 primary radii 

 rather irregularly distributed and usually scarcely differentiated in size from the others 

 (except on brachial valve, where 3 stronger ones generally form a median group) ; 

 a few shorter secondary radii developed irregularly towards margin. Interior of 

 brachial valve with pair of large subtriangular muscle-scars about two-thirds the 

 length of valve, with raised edges, and separated by high sharp median septum 

 extending to weak concentric submarginal ridge formed by row of large pustules at 

 outer ends of short radial ridges. 



Dimensions. — Average length, 4'0 mm. ; average width, 6'0 mm. 



Horizon. — Whitehouse Group. 



Locality. 1 — Whitehouse Bay. 



Remarks. — This shell is distinguishable from the others by the subequal strength 

 and irregular development of the radii, and especially by the internal characters of 

 the brachial valve. No good interiors of the other valve have been observed. 



& 



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



(Plate XV, figs. 4-11.) 



1870. Leptsen't sericea, var. rhombica, M'Coy, Davidson (pars), Mon. Brit. Foss. Brack., vol. iii, pt. vii, 



p. 325 (non pi. xlviii, figs. 20-22). 

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

 ? 1 8 o 2 . Leptxna quinquecostata, M'Coy (pars), Syn. Paleeoz. Foss. Woodw. Mas., p. 236, pi. m, fig. 32. 



Shell transversely semielliptical to subtriangular, widest along hinge-line, concavo- 

 convex, compressed ; cardinal angles not produced, subrectangular or less than right 

 angles. Pedicle-valve moderately convex, most so in middle line, subcarinate or mow 



