THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 535 



this species is a distorted left valve, not altogether free from matrix ; but the sculpture 

 of the valve is so remarkable that the account could not be omitted. I suppose the 

 specimen to be a left valve, in which case the forward projection of the antero-superior 

 angle, as shown by the curvature of the lines of the ornament, is a very curious 

 feature. 



The shell has much the same ornament as C. planulata, Conrad, described by Salter 

 from the Lower Ludlow rocks of Dudley (Mem. Geol. Surv. Gt. Britain, vol. ii., pt. i., 

 p. 349, pi. xxiii. figs. 2, 3, 4). 



Genus Dexiobia, Winchell, 1863. 



Dexiobia, Winchell, 1863, Proc. Acad. Nat. Sc. Phil, p. 10. 

 Miller, 1889, N. Amer. Geol. and Pal, p. 478. 



Generic Description. — " It is inequivalve, inequilateral, beaks separated by an 

 undefined area. Right valve very ventricose, with a very prominent umbo, and a 

 produced, incurved beak, inclined forwards. Left valve much less inflated, with a 

 less prominent beak, scarcely elevated above the dorsal margin. Hinge line more or 

 less extended, straight or slightly bent, edentulous ? furnished with a thickened 

 cartilage plate bearing a linear posterior groove." 



Observations. — Winchell erected this genus for two species which came from 

 the Yellow Sandstone beneath the Burlington group of Iowa, Lower Carboniferous. I 

 have quoted his description in its entirety. Hall (Geol. Surv. Neiv York, Pal., vol. v., 

 pt. i., Lamell., ii. p. xxxvii.) suggests that Dexiobia is a synonym of Cardiopsis, 

 Meek and Worthen, 1861. But the latter is said to be equivalve, and for the present 

 I retain Dexiobia. Hall considers Dualina, Barrande, 1881, to be congeneric with 

 Dexiobia, which must be replaced by the earlier species. The affinities of Dexiobia 

 cannot be discussed without further material, but Dualina has not such a well-marked 

 umbo, and has a different habit of growth. 



Dexiobia elegans, sp. nov. (PI. IV., figs. 28, 28a, 29.) 



Specific Characters. — Shell of moderate size, obliquely gibbose, ovate, acute, very 

 inequivalve. The anterior end is short and low, its margin rounded. The inferior 

 border is very regularly convex. The posterior border curved. The hinge line arched. 

 The umbones are erect, elongate, acutely pointed, and twisted forwards, widely 

 separated and placed anteriorly. The valve is regularly convex. The area wide, but 

 not marked off by any definite line. No lunule or escutcheon. 



Interior. — The hinge line is plain, no cardinal teeth, a single posterior lateral 

 tooth. The adductor scars have not been exposed. 



Exterior. — The surface is almost smooth, but towards the margin obscure, regular, 

 radiating lines are to be seen there, marked anteriorly. 



Dimensions. — Fig. 29, PL IV., measures: antero-posteriorly, 25 mm.; dorso- 

 ventrally, 26 mm. ; elevation of valve, 5 mm. 



TRANS. ROY. SOC. ED1N., VOL. XL VII. PART III. (NO. 18). 78 



