ORDOVICTAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 883 



or less swollen ; beak small, very slightly projecting, not prominent nor swollen, 

 scarcely incurved ; hinge-area triangular, narrow, steeply inclined or in plane of valve. 

 Interior of pedicle-valve with pair of very short broad subrhomboidal diductors 

 traversed by pair of broad divergent vascular trunks bifurcating at about half the 

 length of valve into two strong branches at right angles to trunk and giving off smaller 

 branches to margin ; rest of interior covered with numerous small radiating vascular 

 markings and coarse pustules ; hinge-line with denticulations for half its length on each 

 side of beak. Brachial valve concave ; beak inconspicuous ; hinge-area narrow, at 

 right angles to plane of valve ; interior with muscular platform bearing thick strong 

 median ridge, extending nearly half the length of valve and rising abruptly at front 

 end into blunt spine ; muscle-scars large, broad, flabelliform, each divided longitudinally 

 by ridge into subequal halves ; inner edges of muscle-scars raised ; rest of interior 

 covered with radial lines of more or less fused or elongated pustules. Surface of shell 

 ornamented with 10-18 equidistant primary radii, with shorter secondary and occasion- 

 ally tertiary ones between them, and 3-8 finer thread-like lines in interspaces ; traces 

 of concentric rugse near hinge-line rarely present. 



Dimensions. — Width, 25-27 mm. ; length, 15-17 mm. 



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



Localities. — (.1) Craighead, Minuntion, Colmonel 1 ; (2) Balclatchie, Ardmillan. 



Remarks. — This variety of PI. sericea occurs typically at Craighead and differs 

 from the variety rhombica, M'Coy, # the type of which came from Mathyrafal, by its 

 subcarination, its less numerous radii, the remarkably developed internal septum of 

 the brachial valve, and the much less swollen and non-incurved beak of the pedicle- 

 valve. The muscle-scars of the brachial valve resemble those in the shell attributed 

 by Davidson to PI. sericea from Llanfyllin and Desertcreat.t No interiors of the 

 pedicle-valve from Craighead have been observed, the description of the internal 

 characters of the pedicle-valve here given being based on specimens from Bal- 

 clatchie and Ardmillan, in which the shell is less subcarinate and more uniformly 

 rounded ; these characters closely agree with those of an example referred to PI. 

 sericea from Haverfordwest, figured by Davidson. J 



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



(Plate XV, figs. 12-17.) 



1883. Leptxna sericea, Sowerby, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 227. 



Shell semicircular, plano-convex, compressed, flattened, widest along hinge-line, 

 cardinal angles subacute at 60°-75°, slightly alate, pointed. Pedicle-valve very 

 slightly convex, most so along middle ; beak small, acute, not swollen, scarcely project- 

 ing, very slightly incurved ; hinge-area very narrow, steeply inclined to plane of valve ; 

 interior with pair of very small short divergent triangular diductors about one-fifth 



* M'Coy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 239. 



t Davidson, op. cit., vol. iii, pi. xlviii, figs. 16, 18 (non cet.). % Ibid., pi. xlviii, figs. 19, 19a. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART IV (NO. 26). 127 



