SEDGWICKIA SCOTICA. 283 



Sedgwickia scotica, sp. nov. Plate XXVI, figs. 4 — 7. 



Specific Characters. — Shell of moderate size, inequilateral, quadrately ovate, 

 tumid in the anterior two thirds, but compressed behind. The anterior portion 

 of the valve is short but deep, its. border regularly and broadly rounded, forming 

 a continuous curve with the upper and lower margins. The inferior border is 

 extended, convex, more so at each extremity. The posterior border is narrow, 

 obliquely truncate from above downwards and backwards, making obtuse angles 

 with the upper and lower margins. The hiDge-line is arched, but is extended, erect, 

 and nearly straight behind. The umbones are large, gibbose, elevated, contiguous, 

 and not twisted forwards, and situated in front of the centre of the hinge-line. 



The umbonal swelling rises gradually from the convexity of the shell in front, 

 but is limited behind by a line which passes downwards and backwards 

 obliquely to the postero-inferior angle. Behind this line the shell is rapidly com- 

 pressed and somewhat expanded. 



There is neither lunule nor escutcheon. 



The greatest gibbosity of the valves is subumbonal. 



Interior. — The anterior adductor muscle-scar is small and shallow, situated just 

 within the margin of the shell at the antero-superior angle. The posterior adductor 

 scar, shallow and almost obsolete, is placed in the hollow of the dorsal slope. 

 Hinge as yet unknown. Pallial line entire, situated near the margin of the valve. 



Exterior. — The anterior third of the valve is ornamented with many regular, 

 fine, concentric ridges, which soon become obsolete, so that the posterior two 

 thirds of the valve including the umbones is perfectly smooth. Shell very thin. 



Dimensions. — PI. XXVI, fig. 4, a specimen in the possession of Mr. Adam 

 "VVhyte of Muirkirk, from the McDonald Limestone of that district, measures — 

 Antero-posteriorly . . . .44 mm. 



Dorso-ventrally . . . .37 mm. 



From side to side . . . .25 mm. 



Localities. — Scotland : the Lower Limestone series of Beith, and the McDonald 

 Limestone, Muirkirk, Ayrshire. 



Observations. — This species is founded upon a number of specimens in the 

 cabinets of Mr. J. Neilson, Mr. R. Craig, and Mr. Adam Whyte, and bears a closer 

 resemblance to the general form of Schizodus axiiiiformis, Phillips, sp., than any 

 other species of the genus ; the posterior end is more truncate and less pointed, 

 the umbones less prosogyrous than in this species, which possesses a totally distinct 

 ornamentation to that which obtains in Sedgiviclia scotica, and in casts there 

 appear to be no indications that the hinge possessed teeth. 



