THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 541 



position ; the lunule is large and excavated. The escutcheon well developed. Passing 

 from the umbo to the postero-inferior angle is a rounded ridge which separates the 

 convex and sulcated anterior part of the valve from the compressed and almost smooth 

 dorsal slope. The convex portion shows a very obscure oblique compression, which 

 broadens and becomes more marked near the lower border. 



Interior. — The pallial sinus appears to be entire, and there is no shelly process 

 posterior to the anterior adductor muscle scar. Hinge plate thin and linear posteriorly. 



Exterior. — The anterior two-thirds of the valve is ornamented with somewhat 

 irregular, rugged, concentric folds and rugae, which, owing to the peculiar gibbosity of 

 the valves, are oblique to the long axis of the shell. These rugae and sulci become 

 obsolete on the dorsal slope. 



Dimensions. — Fig. 16, PI. V., measures: antero-posteriorly, 35 mm.; dorso- 

 ventrally, 17 mm. ; thickness of valve, 6 mm. 



Localities. — Bargany Point Burn, Penkhill, Cuddystone Glen, Camregan Wood, 

 Upper Llandovery ; Woodland Point, Newlands, Middle Llandovery ; Mulloch Hill, 

 Lower Llandovery ; Star-fish bed, Upper Bala. 



Observations. — The material from which I have described the species is large in 

 amount, but most of the examples are crushed into various strange shapes, and the 

 various details have been gathered from several specimens. 



The appearance of the shell agrees closely with the figure of Cypricardia ? undata, 

 Sowerby, in the Silurian System. The figure is poor and description meagre. The 

 species is of interest because it shows the extinction of one of the most characteristic 

 features of the genus Grammy sia, i.e. the oblique fold and sulcus in the anterior part 

 of the valve. But Hall (Pal. Neiv York, vol. v., pt. i., Lamell., vol. ii.) shows that 

 both groups existed side by side in the Devonian beds of that State. The form of 

 G. undati approaches closely to the Sanguinolites of the Carboniferous rocks, but seems 

 to be distinctly separated by some internal characters. In shape and habit, S. omaliana, 

 of the highest beds of the Carboniferous limestone of Derbyshire, has a close resemblance 

 to the Ordovician shell. Sowerby referred this species to M'Coy's Orthonota in his 

 second work ; but Hall points out (op. supra cit., pt. xxx., xxxi.) that Grammysia 

 and Orthonota are quite distinct, which is undoubtedly the case. 



G. undata attained a much larger size than the measured example ; but as all the 

 more adult examples are crushed, the true dimensions could not be obtained. 



Grammysia scotica, sp. nov. (PI. V., fig. 12.) 



Specific Characters. — Shell of moderate size, transversely rhomboidal, very 

 inequilateral, moderately gibbose. The anterior end short, its margin rounded. The 

 inferior border almost straight, the posterior border bluntly rounded. The hinge line 

 almost straight. The umbones are small, raised, incurved, and placed far forwards, 

 near the anterior end. Lunule and escutcheon^present, the latter separated from the 



