ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 851 



Orthis (DalmaneUa) Jederata, sp. nov. 

 (Plate IX, figs. 16-22 ; variety, figs. 23-27.) 



1883. Orthis testudinaria, Dalman, Davitlson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 pp. 178, 226. 



Shell subcircular to subquadrate or subelliptical, plano-convex to unequally bi- 

 convex ; hinge-line equal to or rather less than width of shell ; cardinal angles rect- 

 angular or obtuse. Pedicle-valve convex, subcarinate ; beak elevated, pointed, some- 

 what incurved ; hinge-area triangular, high, large, steeply inclined or nearly in plane 

 of valve. Brachial valve nearly flat or gently convex, with shallow median sinus 

 widening anteriorly, sometimes subangular ; beak small, low, inconspicuous ; hinge-area 

 narrow. Interior of pedicle-valve with suboval to subpentagonal well-defined muscle- 

 scar, about one-third the length of the valve, composed of two subtriangular diductors 

 laterally enclosing somewhat shorter adductor impression ; teeth short, stout. In- 

 terior of brachial valve with small oval cardinal process, large stout short crural 

 plates, and median rounded ridge running forwards from thick hinge-plate between 

 two pairs of adductor scars ; posterior adductors small, subcircular, well defined ; 

 anterior adductors smaller, less distinct. Surface of valves ornamented with numerous 

 angular fasciculate ribs, the 14-20 rather coarse primaries dividing into 2 or 3 

 close to beak and each again into 2 or 3 smaller ones at one-half to two-thirds 

 their length ; laterally the ribs curve back slightly ; interspaces angular, deep, equal 

 to or larger than primaries ; all crossed by strong close concentric striation. 



Dimensions. — Length, 5-8 mm. 



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



Localities. — (l) Craighead ; (2) Balclatchie. Variety, Balclatchie, Ardmillan, 

 Dow Hill. 



Remarks. — This small shell is characterised by the fasciculation of the ribs, which 

 is especially well seen in the Craighead examples. The precise mode of division and 

 number of the ribs varies slightly, and in the smaller specimens the ribs are relatively 

 coarser and fewer. Davidson referred these shells to O. testudinaria, but the ribbing 

 and muscle-scar of the pedicle-valve mark them off, and they seem most like O. obtusa, 

 Pander, as interpreted by Gagel,* and the Balclatchie form somewhat closely resembles 

 the shell referred by me to this species from the Tourmakeady Beds in Ireland, t 

 The fasciculation of the ribs recalls 0. hirnantensis, M'Coy4 and 0. edgelliana, Dav.,§ 

 as well as O. argentea, His., as figured by Lindstrom || and Wiman.1T The ribs are 

 coarser in some of the examples from the Balclatchie Beds, and their fasciculation 

 is generally less marked than in those from Craighead, the muscle-scars in both 



* Gagel, Beitr. naturk. Pretiss., vi (1890), p. 33, t. ii, fig. 23. 



f Reynolds, Gardiner and R,EED,.Quart. Journ. Geol. Soc, vol. lxvi (1910), p. 277. 



| Davidson, op. cit., vol. iii, p. 261, pi. xxxii, figs. 5-9. § Ibid., p. 228, pi. xxxii, figs. 1-4. 



|| Lindstrom, Fragm. Silur., p. 26, t. xiv, figs. 12-15. 



1 Wiman, Arkiv). Zool, Bd. iii, No. 24, p. 9, t. i, figs. 20, 21. 



TRANS. ROY. SOC. EDIN., VOL. LI. PART IV (NO. 26) 123 



