ORDOYICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAJS DISTRICT. 875 



diductors embracing broad adductors. Interior of brachial valve with similar coarse 

 irregular radiating rugae, but without concentric submarginal thickening ; cardinal 

 process small, prominent, with stout basal stalk, and with posterior face faintly 

 trilobed (?) ; crura short, blunt, stout, diverging at about 45° to hinge-line ; low 

 median ridge grooved along middle, extending from thick hinge-plate, increasing 

 in height and thickness anteriorly and nearly reaching margin ; adductor scars 

 elongate, oval, contiguous to median ridge, about one-third length of valve. 



Dimensions. — Length, 13—16 mm. ; width, 18-21 mm. 



Horizon.- — Balclatchie Group (including conglomerate). 



Localities. — Balclatchie, Ardmillan. 



Remarks. — This shell cannot be referred to the genus Strophomena, and it is not 

 identical with the true Stroph. ? retroflexa, Salter, the type of which comes from the 

 Chair of Kildare, the relative convexity of the valves being reversed and the ornamen- 

 tation different. The internal characters of the Irish Str. retrojiexa are unknown. 

 In our Girvan shell the crenulation of the hinge-line, the internal rugosities, the 

 characters of the muscle-scars, the apparently trilobed cardinal process, the piano- or 

 concavo-convex shape of the shell and external ornament form a combination of 

 characters which render its generic location difficult. The genera Leptella* 

 Leptsena, Chonetes, and Plectambonites show some points of resemblance, and 

 perhaps Leptella rather than Plectambonites should receive it. 



Genus Plectambonites, Pander. 



The generic name Plectambonites is employed in the manner advocated by Hall 

 and Clarke, t with PI. planissima, Pander, as the type. But it is probable that it is 

 not an homogeneous genus and that it is capable of subdivision. 



Plectambonites Etheridgei (Davidson). 

 (Plate XIII, figs. 27-31.) 



1883. Lepteena Etheridgei, Davidson, Mov. Brit. Foss. Brach., vol. v, Silur. Suppl., p. 170, pi. xii, 

 figs. 11, 12. 



Davidson's figure (op. cit., fig. 12) of the interior of the brachial valve hardly 



does justice to the specimen. The muscle-scars are more triangular in shape and 



more sharply pointed in front than he represents, and they lie on a thickening of the 



shell, highest anteriorly and ending abruptly so as to form a kind of sloping platform ; 



the muscle-scars also diverge slightly, and there is no median septum between them. 



as he incorrectly represents ; there is also a weak submarginal concentric ridge 



outside which the valve bends down rather more steeply. With regard to the 



pedicle-valve, it is not uniformly convex or semicircular, but is decidedly inflated in 



the middle, with somewhat depressed and pointed lateral angles, making an angle of 



45°- 60°, and thus not rectangular; sometimes they are semicylindrical, though 



* Hall and Clarke, op. cit., Brach., i. p. 293. t Hall and Clarke, op. cit, Brach., i, pp. 295-298. 



TltANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 126 



