538 DR WHEELTON HIND ON 



Observations. — It seems to me that Salter's shell, Mytilus platyphyllus, may be con- 

 veniently placed in this genus, the character of which I have quoted from the original 

 source (op. supra cit.). Probably the genus is not far removed from Orthonota ; but 

 it is more modioliform and obliquely gibbose. The American species come from the 

 Hudson River group, on the upper portion of the Lower Silurian of North America. 



Orthodesma platyphyllus, Salter, 1848. (PI. V., fig. 23.) 



Mytilus platyphyllus, Salter, 1848, Mem. Geol. Surv. Gt. Britain, vol. ii., pt. i., p. 364, pi. xx. 

 figs. 13, 14. 



Specific Characters. — Shell transverse, very inequilateral, narrow in front, 

 expanded posteriorly. The anterior end is depressed and prolonged, almost elliptical. 

 The inferior border is elongate, sinuous, concave in front and convex posteriorly. 

 The posterior border is obliquely truncate, the postero-superior angle obtuse, 

 the postero-inferior very bluntly rounded. The hinge line is depressed in front, 

 straight and elevated posteriorly. The umbones are small, compressed, very slightly 

 raised, anterior. Passing obliquely from the umbo to the postero-inferior angle is a 

 well-marked oblique ridge separating the broad expanded and compressed dorsal slope 

 from the body of the valve. Anterior to this ridge, the valve is compressed, very 

 broadly so towards the inferior border. 



Interior.— Not exposed. 



Exterior. — The surface is ornamented with fine, close, concentric lines of growth. 



Dimensions. — Fig. 23, PI. V., measures : antero-posteriorly, 33 mm. ; dorso- 

 ventrally, 17 mm. at the posterior end. 



Locality. — Lower Llandovery, Craighead. South Wales : Trichrug, Llangadoc. 



Observations. — Salter's two figures differ considerably from each other, and it is 

 the first (fig. 13, op. supra cit.) that I consider the Scotch specimen resembles. It is 

 much smaller, but otherwise has the same characters. A single specimen, the right 

 valve, is in the Gray Collection. 



Genus Goniophora, Phillips, 1848. 



Goniophora, Phillips, 1848, Mem. Geol. Surv. Gt. Britain, vol. iii., pt. i., p. 264. 



Orthonotus, M'Coy, 1855, Brit. Pal. Foss., p. 274. 



Goniophora, Barrande, 1881, Syst. Sil. du Centre Boheme, p. 82. 



„ Hall, 1885, Geol. Surv. New York, Pal., vol. v., pt. i., Lamell., ii., p. xxiii. 



,, Beushaiisen, 1895, Abh. Konigl. Pruss. Geol. Landesanstalt, Heft xvii., Die Lamell. 



des rheinischen Devon., p. 196. 



Generic Description. — Shell equivalve, very inequilateral, rhomboidal or 

 trapezoidal ; anterior end short, narrow, and rounded ; posterior expanded, obliquely 

 truncate, and almost straight. Umbones small, much incurved and twisted forwards, 

 placed anteriorly. From the umbo a strong carinate ridge passes downwards and 

 backwards to the postero-inferior angle, dividing the shell into two compressed portions 

 which meet along this ridge. The dorsal slope is hollowed. A well-marked escutcheon. 



