ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 901 



to the primaries is intercalated at half their length, but all the primaries and 

 secondaries are equidistant on the margin. 



Dimensions. — Length, 14 mm. ; width, 24 mm. 



Horizons. — (l) Penkill Group ; (2) Camregan Group. 



Localities. — (l) Penkill; (2) Bargany Pond Burn. 



Remarks. — This rare shell must be referred to Strophonella, audit agrees in many 

 respects, such as ornamentation and general internal characters, with Stroph. euglypha* 

 but it is more transverse and the muscle-scar in the pedicle-valve is smaller. The 

 difference in the ribbing distinguishes it at once from Str. antiquata, var. woodland- 

 ensis, and the shell is also shorter, but more transverse, and the cardinal angles more 

 pointed. Some examples of it were labelled by Davidson Strophomena antiquata. 



Strophonella undata (M'Coy). 

 (Plate XVIII, figs. 15-18.) 



1846. Orthis undata, M'Coy, Syi>. Sihtr. Foss. Ireland, p. 36, pi. iii, fig. 21. 



1852. Strophomena deltoidea, var. /?, undata, M'Coy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 234, pi. ih, 



figs. 38, 39. 

 1870. Strophomena deltoidea, var. [3, undata, M'Coy, Davidson, Mon. Brit. Foss. Brack., vol. iii, 



pt. vii, p. 295, pi. xxxix, figs. 23, 24. 

 1883. Stro}ihomena deltoidea (Conrad), Davidson, ibid., vol. v, Silur. Suppl., pp. 199, 226. 



The type of this species was obtained from Ireland, and both M'Coy and Davidson 

 came to regard it as merely a variety of Raf. deltoidea, but apparently they were 

 ignorant of its internal characters. Amongst the specimens from the Starfish Bed 

 which i would refer to this species, are casts of the interior. The pedicle- valve is 

 concave and geniculated, not convex as in R. deltoidea; the disc is slightly convex 

 and is as long as wide, and the closely placed rather narrow rugse on it meet the 

 hinge-line at right angles ; the valve is geniculated at about two-thirds its length ; the 

 hinge-area is strongly striated and the hinge-line coarsely denticulated ; the muscle- 

 scar is well defined, transversely subcircular, about one-third the length of the valve, 

 and the large fiabellate diductors are radially ridged ; the ovarian areas are coarsely 

 pustulate. In the brachial valve the disc is usually depressed and slightly concave 

 and the beak is inconspicuous ; internally the cardinal process shows two lobes widely 

 diverging and grooved on their posterior face ; the thickened rhomboidal hinge-plate 

 is produced as a low pointed ridge passing into a thin low median septum between 

 faintly impressed adductors. The radial ornament on both valves is almost the same 

 as described and illustrated by M'Coy, but the primary radii are less distinct. 



The true generic reference of this shell must be to Strophonella rather than to 

 Strophomena, on account of the denticulation of the hinge-line, and a comparison 

 with M'Coy's figured specimens indicates that it should be placed in his species 

 Str. undata. Externally it is practically indistinguishable from Raf. deltoidea, but 

 the internal characters and reversed convexity of the valves forbid us to associate 



* Davidson, op. cit., iii, p. 288, pi. xl, figs. 1-5 (especially figs. 4, 5). 



