ALLORISMA VARIABILIS. 425 



Specific Characters. — Shell of medium size, transverse, very inequilateral, tumid 

 in front, compressed, and somewhat narrowed from above downwards posteriorly ; 

 the whole valve is gently curved from before backwards, the upper border being 

 slightly concave, and the lower margin convex. 



The anterior end is short, tumid, separated from the posterior by a well- 

 marked constriction, and much cut away at the expense of its antero-superior 

 angle. Its margin, bluntly rounded, passes downwards and backwards into the 

 inferior border, which is convex, but sinuous in its anterior third, and rises 

 posteriorly more quickly, to pass into the narrowed and rounded posterior margin. 

 The superior border is much shorter than the length of the valve ; much depressed 

 in front, but rises gradually towards its posterior end to form an obscure obtuse 

 angle with the posterior border. The umbones are small, incurved, pointed, and 

 contiguous ; only slightly raised and moderately tumid ; not marked off from the 

 valve in front, but the anterior edge of the shell is continuous with the anterior 

 edge of the umbo, which is placed very far forwards, and much excavated in 

 front by a large, elongate, depressed lunule. The escutcheon is very large and 

 broad, but gradually narrowed and elevated till it vanishes at the posterior 

 margin. It is marked off from the dorsal slope by a raised, angular, curved 

 ridge. 



The valve is obUquely convex, with a deep and accentuated sinus, which passes 

 from the umbo to the inferior margin, becoming broader and deeper as it passes 

 downwards. The dorsal slope is compressed, and is concave just below the 

 postero-inferior angle, and crossed by obsolete radiating ribs. 



Interior. — The anterior adductor muscle-scar is small and obscure. The 

 posterior is large and shallow, situated below the hinge-line, remote from the 

 posterior end. The pallial line is faint, but deeply sinuate. The inner surface of 

 the valve is marked with irregular concentric grooves and rug^e. 



Exterior. — The surface is ornamented with concentric grooves and ridges, 

 which are somewhat irregular in front, e. g. a rib may split and unite again. 

 These rugge are deeply indented by the oblique sinus, and there is an approach to 

 angularity where the rugse are bent up to pass over the dorsal slope. The ribs 

 are small, much closer, and more irregular towards the lower margin. 



The shell was covered with radiating rows of small tubercles, especially over 

 the dorsal slope. Shell very thin. 



Dimensions. — PI. XLVIII, fig. 1, a shell from the Calderwood Limestone, 

 Kilbride, measures — 



Antero-posteriorly . . . .70 mm. 



Dorso-ventrally . . . .33 mm. 



From side to side . . . .25 mm. 



Localities. — England : the Limestone of Lowick, Northumberland. Scotland : 



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