OEDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 877 



septum or ridge only developed in front and not continued back to cardinal plate ; 

 adductors weakly impressed in these areas, flabelliform, divergent, extending about 

 one-third length of shell. Interior of shell finely punctate in radial lines. External 

 surface ornamented with 5-7 straight radiating primary radii regularly disposed (and 

 occasionally shorter secondary radii), with fine radial striae between them. 



Dimensions. — Length, 5 '5 mm. ; width, lO'O mm. 



Horizon. — Balclatchie Group. 



Locality. — Ardmillan. 



Remarks. — This shell is undoubtedly distinct from the Balclatchie Leptella Grayse 

 and from the Craighead examples of that species (see p. 873), which were wrongly 

 referred by Davidson to L. Uandeiloensis (op. cit., -pi. xii, figs. 27-29). His description 

 of the internal characters and of the external ornamentation of L. Uandeiloensis is 

 an attempted combination of the two, and therefore is unsatisfactory. We cannot, 

 indeed, refer these two species to the same genus, and the typical L. Uandeiloensis 

 from Ardmillan must be placed apparently in Plectambonites. Hall and Clarke # 

 figure just such an internal concentric ridge and pair of saucer-shaped areas in the 

 brachial valve of the shell from the Trenton Limestone referred by them to Plect- 

 ambonites sericea. The muscle-scars and other internal characters seem to forbid us 

 referring our shell to the genus Plectella,\ in spite of many points of resemblance. 



Plectambonites quinquecostata (M'Coy). 

 1846. Orthis quinquecostata, M'Coy. Syn. Silur. Fuss. Ireland, p. 33, pi. iii, fig. 8. 



It may be doubted whether typical examples of M'Coy's species occur in the 

 G-irvan district, but there are undoubtedly several forms which must be regarded as 

 varieties ; and as they are characteristic of different stratigraphical horizons and 

 seem to be limited in their vertical distribution, they are considered worthy of varietal 

 designations. The following varieties have been recognised : — 



Plectambonites quinquecostata (M'Coy), var. nov. balclatchiensis. 



(Plate XIV, figs-. 4-9.) 



1870. Le-ptxna quinquecostata, M'Coy, Davidson (pars), Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 322, 

 pi. xlviii; figs. 26, 27 (non figs. 23-25). 

 ? 1883. Leptxna quinquecostata, M'Coy, Davidson, ibid., vol. v, Silur. Suppl., p. 169, pi. xii, fig. 13. 



Shell transverse, nearly or quite twice as wide as long, semicircular to semi- 

 elliptical ; cardinal angles acute, sometimes slightly produced. Pedicle-valve uni- 

 formly and gently convex, rarely depressed or flattened towards lateral angles ; beak 

 small, acute, not swollen, slightly incurved, scarcely projecting above hinge-line. 

 Brachial valve flat or gently concave. Surface of pedicle-valve with 3 main 



* Hall and Clarke, op. cit., Brach., i, pi. xv, fig. 27. 



t Lamanskt, Mem. Com. Ge'ol. St Pe'tersbourg (N.S.), vol. xx, 1905, p. 156. 



