ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 819 



Trematis siluriana, Davidson. 

 (Plate IV, fig. 11.) 



1866. Discina (Trematis) siluriana, Davidson, Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 71, pi. vi, 

 figs. 8, 8a, 8b. 



This species, of which the type comes from the Bala beds of Horderley, was not 

 recorded by Davidson from the Girvan district. Only three imperfect specimens are 

 known to me, and these occur in Mrs Gray's collection. They show the characteristic 

 radiating lines of transversely subquadrate to subcircular pits lying between the 

 radial ridges, and one specimen consists of the pedicle-valve which has not been 

 previously described in this species. This valve forms a low depressed cone with a 

 narrow triangular fissure running inwards from the margin to the subcentral apex, 

 and it resembles in these respects the pedicle-valve of Tr. millepunctata, Hall, as 

 figured by Hall and Clarke.* 



Dimensions. — Diameter about 20 mm. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Genus Schizocrania, Hall and Whitfield. 



Schizocrania asteroidea, sp. nov. 



(Plate IV, figs. 12-14.) 



1851. Orbiculal crassa, Hall, Salter, Proc. Geol. Soc. London, vol. vii, p. 175, pi. x, figs. 3, 4 ; p. 151, 

 woodcut 7. 

 ? 1866. Discina crassa (Hall?), Davidson, Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 69, pi. vi, fig. 7. 

 ? 1883. Discina crassa, Davidson, op. cit, vol. v, Silur. SuppL, p. 224. 

 Non 1847. Orbiculat [Obolella] crassa, Hall, Falmont. New York, i, p. 290, pi. 79, fig. 8. 



Shell oval ; brachial valve gently convex, most so just behind centre ; apex sub- 

 marginal, low, close to posterior margin, blunt, slightly incurved. Surface covered 

 with fine rounded equal and equidistant thread-like lines, increasing in number by 

 interpolation, and curving back slightly on each side. Pedicle-valve unknown. 

 Interior of brachial .valve with a pair of elongate closely placed posterior muscle- 

 scars converging and narrowing to apex of shell, and with a weak median ridge 

 running forwards from apex. 



Dimensions. — Length, 10'3 mm. ; width, 8'0 mm. ; height, 2'2 mm. 



Horizon. — (l) Drummuck Group (Starfish Bed) ; (2) Whitehouse Group? 



Locality. — (l) Thraive Glen ; (2) Whitehouse Bay. 



Remarks. — This shell resembles Schizocrania jilosa (Hall) t in its shape and 

 ornamentation, and in the muscle-scars of the brachial valve, so far as it is known. 

 The specimen figured by Davidson from Penwhapple Glen as Discina crassa, Hall (?) 

 (and of which the occurrence at Thraive was subsequently recorded), is probably the 



* Hall and Clarke, Palxont. New York, vol. viii, Brach., i, p. 138, pi. ive, fig. 8. 

 t Hall and Clarke, ibid., p. 142, pi. ivo, figs. 22-29 (? 30). 

 TRANS. ROY. SOC. EDIN, VOL. LI, PART IV (NO. 26). 119 



