196 CARBONIFEROUS LAMELLIBRANCHIATA. 



the anterior four fifths; it then may become straight. The posterior border is 

 very narrow, and is acutely rounded. The dorsal border is rounded and short in 

 front, produced and straight behind, the latter portion being at a lower level than 

 the anterior. 



The umbones are small and inconspicuous, incurved and non-contiguous. They 

 are situated at about the junction of the anterior and middle thirds of the 

 hinge-line. In front of the umbones is a small elongate elliptical lunette, and 

 posterior to them is an elongated, comparatively broad escutcheon, which is 

 marked off by an acutely edged fold arising from the apex of the umbo ; and, 

 curving at first outwards and backwards, soon passes inwards and below the 

 level of the hinge edge, terminating at the extreme posterior point of the shell. 

 The escutcheon is divided into two parts by obscure lines, which arise from the 

 edge of the ridge, and pass obliquely inwards and backwards to meet in the 

 middle line. The portion in front of these lines is diamond-shaped and concave ; 

 posterior to them the upper edge of the valve rises in the middle line, so that this 

 part is roof-shaped, but slightly hollow at the sides. The valves are gently and 

 evenly convex ; the greatest convexity is at a point midway on the vertical line 

 which passes from the umbo to the ventral border. 



Interior. — The hinge-plate is thickened, and consists of two portions, anterior 

 and posterior, separated by a pit for the internal cartilage, situated immediately 

 beneath the umbones. The anterior portion is curved, convex upwards, and is 

 set with a number of vertical V-shaped teeth having the apex of the V directed 

 towards the umbo. The anterior teeth are larger in front, and diminish in size as 

 they approach the centre. The posterior portion also consists of numerous 

 similarly shaped, teeth, but arranged with their apices pointed forwards. They 

 are larger in the middle than at either end, and do not occupy the w T hole of the 

 posterior part of the hinge-plate, the hinder half being edentulous. This part 

 is gradually thinned and twisted upwards to form a very thin and delicate edge 

 to the produced portion of the valve. 



The scar of the anterior adductor muscle is shallow and not very conspicuous ; 

 it is situated at the anterior-superior angle of the shell, and is marginal, and 

 extends some little way backwards along the side of the hinge-plate. It is marked 

 off from the umbo by a slight ridge which runs forwards and downwards. The 

 posterior adductor muscle-scar is narrow and transversely elongate, and situated 

 just below the hinge-line near the posterior end. A ridge extends downwards 

 from the interior of the umbo, shown as a well-marked hollow in casts. On the 

 anterior portion of this are two pit-like hollows, probably from the insertion of 

 the pedal muscles. The pallial line is not sinuated. 



Exterior. — The surface is for the most part ornamented with fine, regular, 

 concentric lines of growth, which terminate behind abruptly at the acute edge 



