ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 869 



Rafinesquina (Playfairia) semiglobosina (Davidson). 

 (Plate XII, figs. 13-20.) 



1870. Strophomena imbrex, var. semiglobosa, Davidson (pars), Mon. Brit. Foss. Brack., vol. iii, pt. vii, 



pi. xli, figs. 5, 6 (non figs. 1-4). 

 1883. Strophomena imbrex, var. semiglobosina, ibid., vol. v, Silur. Suppl., p. 195, pi. xv, fig. 11 (non 



figs. 9,10). 



Shell subquadrate to semielliptical, strongly concavo-convex ; valves closely 



appressed, sometimes subauriculate ; hinge-line equal to (rarely less than) maximum 



width of shell ; cardinal angles usually rectangular. Pedicle-valve swollen, 



moderately to strongly convex, not geniculated but regularly arched ; lateral angles 



sometimes flattened ; beak small, not much incurved ; hinge-area triangular, rather 



narrow, steeply inclined to plane of valve, with triangular delthyrium. Interior of 



pedicle-valve with small subcircular forked muscle-scar, well defined at sides, less 



than one-fourth the length of valve, composed of two flabellate diductors divergent 



anteriorly, with narrow elongate adductor scar between them ; teeth small. Brachial 



valve regularly concave, with inconspicuous beak and narrow hinge-area nearly at 



right angles to plane of valve. Interior of brachial valve with strong submarginal 



thickening coarsely corrugated radially ; cardinal process large, thick, prominent, 



bilobed, deeply cleft to base, with grooved subtriangular face to each lobe ; crural 



plates stout, thickened, continued round posterior adductors ; muscle-scars deeply 



sunk ; posterior adductors transverse, subcircular ; anterior adductors elongate, 



forming much narrower less-defined scar in front. Ovarian areas closely pitted. 



Shell finely punctate. Surface of shell covered with 30-50 primary thread-like 



regular equidistant equal riblets, with usually a secondary one arising in each 



interspace at half their length midway between them ; primaries and secondaries 



all equal in strength and equidistant on margin ; 3-7 much finer radial lines in 



each interspace between primaries and secondaries ; short concentric corrugations 



near hinge-line sometimes present. 



Dimensions. — 



(1) (Davidson's pi. xv, fig. 11.) (2) 



Length ... 23 24 mm. 



Width ... 27 31 mm. 



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

 Localities. — (l) Craighead, Bogang, Minuntion, Aldons ; (2) Balclatchie (mud- 

 stones and conglomerate), Ardmillan. 



Remarks. — This shell is so distinct from the Wenlock Raf. imbrex, var. semiglobosa, 

 that it certainly should be specifically separated, though Davidson only put it as 

 another variety of R. imbrex, in spite of remarking upon the differences between 

 them. Our shell much resembles Raf. incrassata, Safford,* as regards the interior 

 of the brachial valve, but has much- smaller diductors in the opposite valve. The 



* Hall and^ Clarke, op. cit., Brach., i, pi. viii, figs. 1-5. 



