118 INTRODUCTION. 



Mytilus, sp., Fischer. 



Pboducta, Conybeare, Phillips, Kcenig, J. de C. Sow., M'Coy. 



Strophomena (part), Bronn (not Rafinesque). 



Pinna, sp., Phillips. 



Pecten, sp., Eichwald. 



Lima, sp., V. Buch. 



Shell variable in shape, inequivalve, transverse or elongated, furnished with lateral 

 auricular expansions ; larger or ventral valve regularly convex, geniculated, or perpendi- 

 cularly incurved ; hinge line straight, commonly shorter than the width of the shell ; 

 area very narrow, or no true area, but the cardinal edge considerably thickened ; 

 beak incurved. Smaller or dorsal valve concave, following the curves of the other valve. 

 Exterior variously costated or striated, sometimes decussated by concentric lines of growth ; 

 from the striae arise innumerable small closely packed tubular spines, especially abundant 

 on the auricular expansions ; in other species they are few in number, large, strong, and 

 irregularly scattered over the surface, or arranged in regular or irregular rows near the 

 cardinal edge ; valves unarticulated, probably kept in place by the fibrous membranes and 

 powerful muscular system of the animal. 



In the interior of the larger valve a narrow mesial ridge separates the two elongated 

 ramified muscular scars left by the adductors ? (hepatic impressions of V. Buch) ; im- 

 mediately under, but outside these, are seen other two very deep longitudinally striated, 

 subquadrate impressions, probably formed by the cardinal muscles, and more or less 

 widely separated by a mesial elevation or crest, and lower down more towards the 

 centre of the shell two other deep concave sub-spiral depressions are visible,^ all the 

 remaining inner surface of the larger valve not occupied by the muscular scars is indented 

 by a multitude of small depressions, increasing in number under the auricles ; in the 

 centre of the hinge line of the smaller valve, a prominent trifid cardinal process afforded 

 attachment to the cardinal muscles, under this a narrow longitudinal ridge extends to 

 more than half the length of the valve, and becoming more elevated towards its extremity ; 

 on either side, are seen the ramified scars left by the adductors, and corresponding to 

 those situated in a similar position in the other valve ; each scar is double, the two 

 portions being separated by the commencement of the great vascular trunks (see PI. IX, 

 f. 219). Outside, and in front of the muscular scars, are two elongated or reniform im- 

 pressions ; their surface is smooth, and they are bounded by ridges, which, after dividing 

 the adductors proceed in an outward oblique or almost horizontal direction, when turning 

 abruptly backwards they terminate at a short distance from their origin ; between the 

 vascular impressions, and close to the muscular scars, exist two prominences, one on each 

 side of the cardinal ridge ;* the internal surface of the smaller valve is covered with innu- 



1 Supposed by V. Buch, De Koainck, King, and others, as intended to afford space for spiral or 

 labial appendages. 



^ These are supposed to indicate the origins of the spiral arms ; in which case Productus differs from 



