820 DR F. R. C. REED ON THE 



same species. But Hall's Orbicula ? [Discina] crassa is referred by Schuchert* 

 to the genus Oboletta, and is a Lower Cambrian species ; and Walcott t likewise 

 places it in this genus. 



Genus Orbiculoidea, D'Orbigny. 



Orbiculoidea Armstrongi, sp. nov. 



(Plate IV, figs. 15-17.) 



Shell circular or slightly subcircular. Pedicle-valve gently convex, low conical ; 

 apex subcentral ; interior with pedicle-tube extending rather more than half the 

 distance from centre to posterior margin. Brachial valve low conical, with apex ex- 

 central, situated at about one-third the diameter or less from posterior margin. Surface 

 marked with strong regular rounded concentric ridges of rather unequal size. 



Dimensions. — Average diameter 8 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Remarks. — This species is undoubtedly very close to Discina perrugata, M'Coy ,j 



the type of which came from the Llandovery beds of Kilbride, Ireland. But I am 



not convinced that it is identical, especially as it comes from a different horizon. 



M'Coy says that the apex is situated at one-third the diameter from the margin ; 



this would apply only to the brachial valve. Davidson does not record Orbic. [D.] 



perrugata from the Drummuck beds, but he states § that it occurs at Balclatchie. 



1 have not, however, seen any specimens attributable to it from this locality. It may 



be named Armstrongi, after one of the authors of the Catalogue of Western Scottish 



Fossils, 1876. 



Orbiculoidea shallochensis, sp. nov. 



(Plate IV, figs. 18-20.) 



1883. Discina perrugata, Davidson (pars), Mori. Brit. Foss. Brach., vol. v, Silur. Suppl., pp. 210, 224 

 (non pi. xvii, fig. 47). 



Shell circular to subcircular. Pedicle-valve low conical, with sharp central apex ; 

 interior with pedicle-tube extending nearly three-fourths the distance from centre to 

 posterior margin ; edge of valve somewhat flattened or thickened. Brachial valve 

 very low conical, more or less flattened ; apex very excentral, situated at about one- 

 fourth the diameter from posterior margin. Surface with numerous fine concentric 

 close rounded lines. 



Dimensions. — Length, 9"0 mm. ; width, 8 '5 mm. 



Horizon. — Whitehouse Group. 



Locality. — Whitehouse Bay, Shalloch Mill. 



Remarks. — The finer surface-ornamentation and the longer pedicle-tube and more 

 excentnc apex of the brachial valve distinguish this species from O. Armstrongi. 



* Schuchert, Bull. 87, U.S. Geol. Surv. (1897), p. 373. f Walcott, Camb. Brack, 1912, p. 592. 



+ M'Coy, Syn. Silur. Foss. Ird., 1846, p. 24, pi. iii, fig. 2. 



Sj Davidson, Mon. Brit. Foss. Brach., vol. iii, pt. vii, p. 65, pi. v, fig. 23. 



