THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 501 



Postero-superior angle obtuse. The umbones are not well marked off from the valve, 

 blunt, and all but terminal. Passing from the umbo to the lower margin is a blunt, 

 subangular oblique ridge separating the narrow compressed triangular anterior from 

 the more gradually compressed and expanded subalate posterior end. 



Interior. — Hinge not exposed. Anterior adductor muscle scar small, subumbonal, 

 and immediately within the anterior end. 



Exterior. — Surface almost smooth ; but under the microscope fine concentric lines 

 of growth are to be seen, with here and there one more strongly impressed. 



Dimensions. — Fig. 3, PI. III., a bivalve example, measures : antero-posteriorly. 

 33 mm. ; dorso-ventrally, 24 mm. ; from side to side, 16 mm. 



Localities. — Mulloch Hill, Lower Llandovery ; Woodland Point, Middle Llandovery. 



Observations. — Three specimens in the collection I refer to Gosseletia on account of 

 the general shape ; but I have not been able to see the hinge plate. Gosseletia antiqua 

 is much more transverse and oblique than G. triquetra of the Hamilton group of New 

 York, and the anterior end is not so adpressed. 



Euthydesma, Hall, 1885. 



Euthydesma, Hall, 1885, Pal. New York, vol. v. p. 32. 



,, Miller, 1889, North American Geol. and Pal., p. 480. 



„ Beushau-;en, 1895, Abli. Konigl. Geol., p. 316. 



Specific Characters. — Shell inequilateral, equivalve, broadly subovate, with a sub- 

 alate cardinal expansion. Cardinal line straight, anterior end short, surface concen- 

 trically lined ; hinge line marked by a continuous ligamental groove. 



Observations. — I have adopted Miller's description of the genus (op. cit.). One 

 species only, it appears, has been described from North America. Beushausen has also 

 described one species from the Upper Devonian of Oberscheld and Martenberge, near 

 Adorf. I have referred these specimens in the Gray Collection to this genus under the 

 following name : — 



Euthydesma alata, sp. nov. (PI. I., figs. 11, 12.) 



Specific Characters. — Shell below medium size, broadly ovate, moderately com- 

 pressed, the posterior superior angle flattened and expanded. The anterior margin 

 starts close to the umbo, descending for a short distance ; it then curves suddenly out- 

 wards, and then becomes strongly convex and passes with a regular, almost semi- 

 circular curve into the broad, convex lower margin. The posterior border is almost 

 straight and erect. The hinge line very short in front of the umbo, straight and of 

 moderate length posteriorly. The umbones are raised, pointed and gibbose, twisted 

 forwards. 



Interior. — Unknown. 



Exterior is almost smooth, but here and there moderately strong concentric lines 

 denoting interruptions of growth are to be seen. 



