CYPRICARDELLA RECTANGULARIS. 357 



antero-posterior diameter of the valve. The umbones are small, contiguous, and 

 directed forwards, and placed very anteriorly, much elevated above the anterior 

 end, but not raised above the rest of the shell. The lunule is well marked, 

 and the escutcheon elongate and narrow. Passing downwards and backwards 

 obliquely from the umbo to the postero-inferior angle is an obtuse ridge, in front 

 of which the valve is slightly convex, but behind it is rapidly compressed and 

 expanded, forming the dorsal slope. 



Interior. — The anterior adductor muscle-scar is small and round, placed 

 immediately within the antero-superior angle. The hinge of the right valve has 

 a single, central, cardinal tooth, with a socket on either side, a long, anterior, 

 lamellar tooth, and a smooth hinge-plate posteriorly bevelled at the expense of 

 its lower edge. 



Exterior. — The surface is ornamented with fine, concentric, raised lines of 

 growth. 



Dimensions. — Fig. 21, PI. XXXIX, in the collection of Mr. J. Neilson, from 

 Gallowhill, Strathavon, measures — 



Antero-posteriorly . . . .12 mm. 



Dorso-ventrally . . . .8 mm. 



From side to side . . . .5 mm. 



Localities. — England : in a thick bed of calcareous shales, in the Faraday Grill, 

 above the Underset Limestone on the west flank of the Nine Standards, Kirkby 

 Stephen. Scotland : the Upper Limestone series of Orchard, and Williamswood ; 

 the Lower Limestone series of Craigen Glen, Campsie; Gallowhill, Strathavon. 



Observations. — This species, I have every reason to believe, is that described 

 by Young and Armstrong in both their catalogues as Gypricardia rhombea, 

 to which, however, it has not the very slightest resemblance. G. rectangularis is 

 much less strongly marked with concentric ridges than G. concentrica, but is not 

 so smooth as G. Annse ; but the latter is at once distinguished by its square 

 shape, G. rectangularis always being oblong even in the very young. The stages 

 of growth are very well marked on the valves, and the protoconch has the same 

 shape as the full-grown shell. 



Fortunately I have been able to obtain access to a specimen which shows the 

 hinge-plate and part of the interior, fig. 23, PI. XXXIX. This is in the collection 

 of Mr. J. Smith, of Kilwinning. 



Notwithstanding the large size of the escutcheon, no trace of external ligament 

 is visible in the most perfect examples. 



Nucula rectangularis, M'Coy, is preserved in the Griffith Collection of the 

 Science and Art Museum, Dublin. It is a very small and poor specimen, but it 

 certainly has many of the characteristics of the better preserved Scotch examples. 

 M'Coy's description is far from accurate, and I cannot think that his observation 



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