THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 533 



Genus Dualina, Barrande, 1881. 



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Lunulicardium, Salter, 1861, Mem. Geo!. Surv. Gt. Brit., " Geol. Neighbourhood Edinburgh 



p. 139. 

 Dualina, Barrande, 1881, Syst. Sil. dn centre de la Bohhne, I re partie, vol. vi. p. 77. 

 Beushausen, 1895, Abh. Konigl. Preuss. Geol. Landesanslalt, Neues Folge, Heft xvii. p. 298. 



Generic Characters. — Shell inequivalve, inequilateral, triangularly ovate. The 

 anterior end the larger. The urn bones raised, unequal, and not opposite or contiguous. 

 A blunt ridge or rounded swelling passes from the umbo downwards and backwards. 



Interior. — Unknown. 



Exterior. — A well-marked ornament of more or less close radiating ribs seems to be 

 present in all species 



Observations. — The curiously shaped shells described by Barrande under the 

 name Dualina are only known by their external characters. He states that there are 

 three principal characters which distinguish the genus : — 



1 . The inequality of the valves, though it seems that it is sometimes the right and 



sometimes the left which is the more gibbose. 



2. The inequality of the umbones of the two valves ; this is naturally a corollary 



of No. 1. 



3. The more gibbose valve is inclined towards one side. 



Unfortunately, there are only two shells in the collection which can be referred to 

 the genus, and these are single valves, so that nothing more is to be learned of the 

 characters of the genus from the material at hand. 



Dualina elegans, Salter, sp., 1861. (PI. IV., fig. 23.) 



Lunulicardium elegans, Salter, 1861, Mem. Geol. Surv. Gt. Brit., " Geol. Neighbourhood Edinburgh," 

 p. 139, pi. ii. fig. 10. 



Specific Characters. — Shell small, moderately compressed, triangular. The posterior 

 end small and adpressed. The anterior and lower borders rounded; the posterior 

 obliquely truncate. The hinge line apparently short and arched. The umbones are 

 raised, pointed, subcentral. Passing from the umbo to the postero-inferior angle is an 

 acute angular ridge, separating a compressed and flattened dorsal slope from the rest of 

 the valve. The interior is not exposed. 



Exterior. — The surface is ornamented by numerous fine, regular, radiating ridges, 

 which are almost obsolete on the flattened dorsal slope. Here and there these ridges 

 are crossed by rugse and lines of growth. 



Dimensions. — Fig. 23, PI. IV., measures: antero-posteriorly, 16 mm.; dorso- 

 ventrally, 15 mm. 



Locality. — The Upper Llandovery of Penwhapple Glen, Graptolites priodon bed. 



Observations. — Salter's species has a strong resemblance to D. secunda, Barrande, 

 from Stage E in Bohemia. A single specimen occurs in the Gray Collection, a right valve, 



