ORDOVICIAN AND SILURIAN BRACHJOPODA OF TJHE GIRVAN DISTRICT. 861 



of the interspaces distinguish them from the variety found in the Drummuck Beds 

 which is described below. It is possible that the species also occurs at Craighead, 

 but the evidence is not quite conclusive. Lindstrom * figures a shell from the 

 Chasmops Limestone of Sweden as a variety of 0. Actonise with a query, which 

 much resembles our Shalloch Mill form ; and G-agel's f shell from the drift of 

 Germany seems to be practically identical. 



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



Localities. — (l) Shalloch Mill ; (2) Craighead. 



Orthis (Nicolella) Actonise, Sowerby, var. nov. asteroidea- 

 (Plate X, figs. 28-35 ; Plate XI, figs. 1, 2.) 

 1883. Orthis Actonise, Sowerby'?, Davidson, Man. Brit. Foss. Brack., vol. v, Silur. SuppL, p. 226. 



Shell semicircular to semielliptical, plano-convex to concavo-convex, somewhat 

 flattened near cardinal angles, about one and a half times to twice as wide as long ; 

 hinge-line straight, as wide as or slightly wider than shell ; cardinal angles 

 rectangular or pointed at about 75°, not produced. Pedicle-valve more or less 

 swollen, strongly convex in middle, somewhat flattened towards cardinal angles ; 

 beak rounded, broad, swollen, incurved, rising well above hinge-line ; hinge-area 

 moderate, lying in plane of valve or steeply inclined. Brachial valve flattened or 

 slightly concave ; beak small, inconspicuous ; hinge-area narrow. Interior of pedicle- 

 valve with broad transverse or short subcircular faint muscle-scar, composed of 

 lanceolate ' median adductor scar, embraced by diductors ; teeth short, small ; apex 

 of beak with shell thickened internally. Interior of brachial valve incompletely 

 known ; cardinal process small, lanceolate ; crural plates small, making very acute 

 angle with hinge-line. Surface of valves ornamented by coarse equal or subequal 

 angular or subangular ribs, occasionally dividing close to beak into two equal or 

 unequal ribs, but usually increasing in number rather irregularly and unsym- 

 metrically by intercalation of a smaller short rib at about one-fourth the distance 

 from the margin ; 24-30 ribs in all on margin. Bibs in brachial valve straight, 

 radiating ; lateral ribs in pedicle-valve curved slightly outwards to cardinal angles. 

 Interspaces angular, equal or subequal to ribs. Concentric striation present. Ribs 

 and interspaces clearly visible over whole interior of shell. 



Average Dimensions. — Length, 15 mm. ; width, 20-22 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — This abundant shell of the Starfish Bed differs from the typical 

 0. Actonise as illustrated by Davidson| by its cardinal angles being non-alate, _by 

 the possession of more numerous ribs, and by the muscle-scar in the pedicle-valve 



* Lindstrom and Angelin, Fragm. Silur., p. 27, t. xii, figs. 44-47 (non t. xiv, figs. 4-9). 



t Gagel, "Bracli. camb. silur. Gesch. Ost-Westpreuss." (Beitr. naturh. Preuss., vi, 1890), p. 31, t. ii, fig. 25. 



\ Davidson, op. cit., vol. iii, p. 252, pi. xxxvi, figs. 5-12 (? 13), 14-17. 



