THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 499 



Genus Myalina, de Koninck, 1842. 



Myalina, de Koninck, 1842, Animaux Foss. Carb. Beige, p. 25. 

 Psilonychia, Ulrich, 1893, Rep. Geol. Surv. Ohio, vol. vii. p. 648. 

 Myalina, Hind, 1897, Mono. Brit. Carb. Lamell. (Pal. Soc), p. 103. 



Generic Characters. — Shell triangular, oblique, mytiliform, expanded at the anterior 

 border, compressed and subalate posteriorly. Umbones terminal. Hinge plate thickened, 

 longitudinally striate. 



Exterior. — Concentric lines of growth well marked, often subimbricate or 

 lamellar. 



Observations. — For a fuller synonymy and description I would refer to my mono- 

 graph {op. supra cit). I cannot but think that Psilonychia of Ulrich is unnecessary, 

 as, according to his definition, there is not one single character mentioned which does 

 not obtain in Myalina. He describes a single species, P. perangulata, which differs 

 in shape from M. prisca, but the figures fully demonstrate the characteristic 

 structure of Myalina. 



A single specimen, which I now refer to M. prisca, is in the Gray Collection. 



Myalina prisca, sp. nov. (PI. II., fig. 10.) 



Specific Characters. — Shell of moderate size, compressed, very inequilateral, 

 triangular ; the anterior end is adpressed and hollowed, and the thickest part of the valve 

 is at the anterior border, but the anterior junction of the valves is so depressed that it 

 lies some way posterior to the border. The anterior border is straight and sloping 

 downwards and backwards, the inferior margin curved ; the posterior obliquely trun- 

 cate, straight above, but rounded below. The hinge line straight, and the longest 

 transverse diameter of the valve. The postero-superior angle well marked and obtuse. 

 The umbones are pointed and prosogyrous, terminal. 



Interior. — The hinge plate is elongate, broad at the umbonal end, and 

 becoming gradually narrower as it passes backwards, hollowed and finely striate 

 longitudinally. 



Exterior. — The surface is ornamented with concentric, somewhat irregular strise and 

 rugse of growth, at times almost subimbricate. 



Dimensions. — Fig. 10, PI. II., measures: antero-posteriorly, 25 mm. ; dorso-ventrally, 

 about 25 mm. 



Locality.— Middle Bala of Shalloch Mill. 



Observations. — A very typical Myalina, with a strong resemblance to M. redesdal- 

 ensis, Hind, a Carboniferous species, but the latter is much more massive and robust. 

 Fortunately, the specimen, a right valve, shows the hinge plate very well. The longi- 

 tudinal strise on the hinge plate are not so strongly marked as in Carboniferous species 

 of the genus. More transverse and with a longer hinge plate than M. perangulata, 

 Ulrich, sp. 



