ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 871 



deltidium covering delthyrium. Brachial valve flat or slightly concave ; beak incon- 

 spicuous ; hinge-area narrower than in opposite valve, and deltidium smaller. Surface 

 of valves marked with 7-12 fine straight thread dike primary radii, equidistant and 

 of equal strength, separated by wide flat interspaces ; at half the length and midway 

 between the primaries an equally strong but shorter secondary radius is intercalated, 

 and 6-8 very fine delicate radial striae on each side of it filling up the interspace. 

 Closely placed concentric ruga3 present on both valves, usually fine, sometimes 

 coarse, continuous on pedicle-valve only near hinge-line, which they meet at acute 

 angles, and usually less strongly developed over rest of surface, but always at right 

 angles to primary radii, thus giving an angulated appearance to the ornamentation ; 

 rugse generally stronger and more numerous in adult shells, wider apart or absent in 

 young individuals, but fine concentric wrinkling of surface always present. 



Dimensions. — Length, 2'0-12 mm, ; width, 3'5-23 mm. 



Horizon. — Balclatchie Group. 



Localities. — -Balclatchie, Ardmillan, Dow Hill, Benan Crag ? 



Remarks. — This shell was included by Davidson in his Stroph, corrugatella, but 

 it is a well-marked form limited to the Balclatchie Group, and differs in its more 

 transverse shape, flatness, and ornamentation. The majority of the specimens are 

 small, averaging 10-14 mm. in width. The specimens figured by Davidson from the 

 Balclatchie conglomerate (op. cit., pi. xv, figs. 23, 24, 26) are amongst the largest 

 and have the coarsest rugae. In the finer-grained and shaly beds the specimens have 

 finer rugae. The resemblance to Str. arachnoidea, Tornquist,* is noticeable, but our 

 shells are usually alate and have fewer primary rays and stronger transverse rugae. 

 Gagel's f Stroph, corrugatella, from the Beyrichia Limestone, seems somewhat 

 similar to the Giryan form. The differences separating it from B. brumalis of 

 Balclatchie have been mentioned above. 



Genus Lept^ena, Dalman. 

 Leptsena rhomboidalis (Wilckens). 



(Plate XII, fig. 32 ; Plate XIII, figs. 1-9.) 



1769. Gonchita rhomboidalis, Wilckens, Nachricht. selten Verstemerungen, p. 77, pi. viii, figs. 43, 44. 

 1883. Sfrophomena rhomboidalis (Wilckens), Davidson, op. cit., vol. v, Silur. Suppl., pp. 192, 226. 



This common species ranges through the whole succession of beds in the Girvan 

 area from the Stinchar Limestone Group to the Penkill Group. But the examples 

 from each group have certain peculiar and distinctive features which, though scarcely 

 deserving varietal designations, are worthy of notice. 



(l) (var. a.) The specimens from Craighead, Minuntion, and the Balclatchie con- 

 glomerate are transversely oblong with scarcely any auriculation, the ears which are 

 present in most other examples being absent ; the geniculation of the shell is at 



* Lindstrom, Fragm. Silur., p. 29, t, xiv, figs. 41-43. 



t Gagel, Brack, camb. silur. Gesch., 1890, p. 46, t. iii, fig. 19. 



