THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 537 



Locality. — Scotland : Star-fish bed, Upper Bala. Wales : Marloes Bay. 



Observations. — Unfortunately, the specimen in the Gray Collection is crushed 

 immediately posterior to the umbo ; the posterior end is foreshortened and expanded. 

 I have, however, very little doubt that it belongs to Phillips' species. Marloes Bay 

 is stated {op. supra cit.) to belong to the middle part of the Llandeilo series. 



0. inornata is much more transverse and has a more distinct linear ornament than 

 O. nasuta. 



Orthonota nasuta, Conrad, sp., 1841. (PL V., fig. 22.) 



Cypricardites 7iasutus, Conrad, 1841, Ann. Rep. New York, p. 52. 



,, „ Emmons, 1842, ibid., p. 403, fig. 4. 



Modiolopsis nasutus, Hall, 1847, Pal. New York, vol. i. p. 159, pi. xxxv. fig. 7. 

 Orthonotus nasutus, M'Coy, 1855, Brit. Pal Foss., p. 275, pi. i. fig. 23. 



Specific Characters. — Shell narrow, elongate, very moderately convex, superior 

 and inferior margins subparallel. The anterior end, much narrower than the rest of the 

 shell, has an elliptical margin. The posterior end somewhat expanded, its margin bluntly 

 rounded. The umbones are small, situated at the anterior fourth of the hinge line. 



Interior. — Unknown. 



Exterior. — The surface is ornamented with concentric lines and rugse of growth, 

 some of which are strong enough to merit the term subimbricating. 



Dimensions. — Fig. 22, PL V., a right valve, measures : antero-posteriorly, 18 mm.; 

 dorso-ventrally, 7 mm. 



Locality. — Scotland : Llandeilo beds, Dowhill. England : Caradoc Sandstone, 

 Horderly West, Shropshire. 



Observations. — This species has been figured and described by M'Cov from a 

 British locality, in the Caradoc Sandstone, a somewhat higher horizon than that whence 

 the Scotch specimen was obtained. Hall states of his specimens, one about the same 

 size as the Scotch : " This is probably the young of the species which appears in the 

 Hudson River group, where it has attained a much greater magnitude." M'Coy's 

 specimen was 32 mm. long. 



Genus Orthodesma, Hall and Whitfield, 1875. 



Orthodesma, Hall and "Whitfield, 1875, Rep. Geol. Surv. Ohio, vol. ii., pt. ii., Pal., p. 92. 

 Miller, 1889, N. Amer. Geol and Pal., p. 497. 



Generic Characters. — "More or less elongate bivalve shells having the hinge line 

 straight and generally extended posterior to the beaks, but contracted or bent beneath 

 or anterior to them. Hinge plate apparently edentulous. Valves united by an 

 external ligament extending to a greater or less distance along the posterior cardinal 

 margin. Posterior muscular scar elongate, ovate, anterior scar small, both faintly 

 marked. Pallial line simple. Shells thin, marked externally with irregular concentric 

 plicae. Type, Orthodesma recta." 



