284 CARBONIFEROUS LAMELLIBRANCHIATA. 



At present this species has only been obtained from the horizon of the Lower 

 or Beith Limestone, of which the McDonald Limestone at Muirkirk is the 

 equivalent, the shells occurring in the shales which are found in connection with 

 the beds of limestone, and not in the limestones themselves. 



Sedgwickia suboebicularis, sp. nov. Plate XXVII, figs. 5 — 8. 



Specific Characters. — Shell transversely suborbicular, compressed, very slightly 

 oblique, inequilateral. The anterior end is short but deep, compressed, its margin 

 regularly rounded, passing without a break into the superior and inferior borders. 

 The lower margin is only slightly convex, the posterior regularly convex, passing 

 into the hinge-line above and the lower margin below with an uninterrupted 

 curve. The hinge-line is gently curved, subparallel with the lower border. The 

 umbones are broad, flattened, raised above the hinge-line, and placed in the 

 anterior third of the shell. The posterior part of the shell is compressed and 

 somewhat expanded. The general curvature of the valve is regular, but slight 

 in amount. 



Interior unknown. 



Exterior. — The surface is ornamented with about twenty-five regular raised 

 lines, which separate as they pass backwards, but become obsolete about the 

 junction of the anterior and middle thirds of the valve. Posteriorly the shell is 

 almost smooth, or with a few obsolete flattened sulci. If the outer layer of shell 

 be removed, the inner portion shows obscure radiating lines. Shell very thin. 



Dimensions. — Fig. 6, PL XXVII, a left valve, measures — 



Antero- posteriorly . . . .36 mm. 



Dorso-ventrally . . . .28 mm. 



Elevation of valve . . . .4 mm. 



Localities. — Scotland: Inverteil Quarry, Linktown, Kirkcaldy; and Potmetal 

 Plantation, Kirkcaldy ; Hope Quarry, Pathhead, Haddingtonshire. 



Observations. — I have founded this species on four specimens in the Collection 

 of the Geological Survey of Scotland, one of which, PI. XXVII, fig. 7, is 

 that of a very young example. All the specimens have the shell preserved, so 

 that it is impossible to say anything about the interior of the valve. 



None of the six species described by M'Coy resemble S. suborbicular is, which 

 is at once distinguished by its blunt, obtusely rounded posterior end, which is 

 larger from above downwards than the anterior. The species is also much more 

 compressed than any other member of the genus. 



Both the specimens from Inverteil, figs. 5 and 6, PI. XXVII, have the 



