ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 797 



Note. — It is to be regretted that the brachiopods from the Ballantrae Group are 

 so rare and poorly preserved that their description is at present unavoidably brief 

 and incomplete. The identifications of species given by previous writers are 

 mostly questionable, but it seems to be the case that nearly all the forms belong to 

 the Atremata. 



DESCRIPTION OF THE SPECIES. 



Genus Micromitra, Meek. 



Subgenus paterina, Beecher. 



Micromitra {Paterina) Davidsoni, sp. nov. 



(Plate I, figs. 1, 2.) 



1870. Acrotreta (?) Nicholsoni, Davidson (pars), Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 343, pi. xlix, 



figs. 38, 39 (non figs. 36, 37, 40). 

 1883. Acrotreta Nicholsoni, Davidson (pars), op. cit., vol. v, Silur. Suppl, p. 213. pi. xvi, fig. 22 (non 



figs. 21, 23). 

 1912. Acrotreta Nicholsoni, Walcott (pars), Cambrian Brachiopoda~ (Mon. U.S. Geol. Surv., vol. li), 



p. 696, pi. Ixxiii, figs. 1/t, \i (non cet.). 



Shell minute, subcircular, with a nearly straight hinge-line and rounded cardinal 

 angles. Pedicle-valve conical, with slightly excentric pointed apex, perforated ; 

 apical angle 100°-110°. Posterior face of pedicle-valve flattened, nearly vertical or 

 steeply inclined, forming triangular false-area, bearing narrow convex triangular 

 pseudo-deltidium in middle third, sunk between rather deep grooves. Surface of 

 shell marked with fine concentric striae. 



Dimensions. — Diameter, 1'2-1'3 mm. ; height, 0'8-0'9 mm. 



Horizon. — Balclatchie Group. 



Locality. — Balclatchie. 



Remarks.— This small shell cannot be referred to the genus Acrotreta, if we 

 follow Walcott's # revised definition of the genus, for the latter has a median 

 vertical groove across the false-area of the pedicle-valve and no pseudo-deltidium. 

 The species from Dobbs' Linn, Moffat, described by Davidson t as Acrotreta (?) Nichol- 

 soni, to which these Balclatchie shells have been referred, differs in having a much 

 lower pedicle-valve, a much wider apical angle, a median groove across the false-area, 

 no pseudo-deltidium, and an impressed line limiting the false-area on each side. Our 

 Girvan species appears to be referable to the subgenus Paterina of the genus Micro- 

 mitra according to Walcott's definition,! and may be especially compared with the 

 type-species P. [Iphidea Billings 1872, non Baly 1865] bella, Billings.§ 



* Walcott, Camb. Brack (Mon. U.S. Geol. Surv., vol. li, 1912), p. 671. 

 t Davidson, Geol. Mag., vol. v (1S68), p. 313, pi. xvi, figs. 14-16. 

 ; Walcott, op. cit., pp. 332, 343. 

 § Ibid., p. 344, pi. ii, figs. 1, la-c. 



