ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 865 



Remarks. — Our Girvan specimens agree completely with specimens of Portlock's # 

 " Strophomena" concentrica from the typical locality in Tyrone, and are closely com- 

 parable with Raf. alternata (Conrad) f from the American Ordovician. The crenulation 

 of the posterior edge of the inner face of the hinge-teeth in the pedicle-valve is well 

 seen in our specimens. It does not seem possible to include this form with the typical 

 " Stropliomena" expansa, Sowerby ,| the types of which came from Moel y Garth 

 and Gaerfawr, near Guilsfield, Montgomeryshire. 1 have not been able to detect 

 the dental crenulations in the Welsh examples, which with the concentric corruga- 

 tions on the surface and the marked submarginal internal ridge in both valves seem 

 together sufficient to separate this Girvan species from the true R. expansa (Sow.). 



Rafinesquina expansa (Sowerby), var. no v. Macallumi. 



(Plate XI, figs. 17-20.) 



1883. Strophomena expansa, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 227, 

 pi. xv, fig. 2 (non figs. 1, 3-5). 



Shell subquadrate to semielliptical, concavo-convex, not flattened but much 

 compressed ; hinge-line equal to width of shell ; cardinal angles rectangular or 

 slightly obtuse ; hinge-line not denticulate. Pedicle-valve very gently convex ; 

 beak small, pointed, scarcely rising above hinge-line ; hinge-area narrow. Brachial 

 valve slightly concave ; beak inconspicuous ; hinge-area narrower than in opposite 

 valve. Interior imperfectly known ; muscle-scars very faint.- Pedicle-valve with 

 very weakly impressed iiabellate diductor scars, about one-fourth to one-third length 

 of valve. Surface of valve covered with 40-60 subequidistant regular straight 

 filiform primary riblets of equal size and increasing in number by intercalation at 

 about half their length and again nearer margin, with 3-7 much finer intermediate 

 thread-like lines between them in interspaces ; whole surface of shell crossed by very 

 delicate concentric striation. Substance of shell finely and densely punctate. 



Dimensions. — Length, 20-29 mm. ; width, 25-44 mm. 



Horizon. — Stinchar Limestone Group. 



Localities. — Minuntion, Doularg, Bogang, Bennane Burn. 



Remarks. — Specimens of this shell were labelled simply Strophomena expansa 

 by Davidson, but they differ from Sowerby's type in the more strongly concavo- 

 convex shape of the shell, the finer ornamentation, and the feeble and smaller 

 diductor scars. The American species Raf. alternata (Conrad), § from the Lorraine 

 Group, bears a close resemblance to them, and we may also draw attention to the 

 similarity of Raf. Winchester ensis-filistriata, Foerste.|| It seems to deserve a 

 distinctive appellation, and we may term it Macallumi, after the well-known 



* Portlock, Geol. Bep. Londond., 1843, p. 452, pi. xxxvii, fig. 1 ; Davidson, op. cit., pi. xlv, fig. 9 (non cet.). 



t Hall and Clarke, op. cit., p. 281, pi. viii, figs. 6-11. 



t Sowerby, in Murchison's Silur. SysL, 1839, p. 638, pi. xx, tig. 14. 



§ Hall and Clarke, op. cit., Brack, i, pi. viii, figs. 6-11. 



II Foerste, Bull. Venison Univ., vol. xvi, art. 2 (1910), p. 42, pi. v, figs. 14, 15. 



