(13) 



5. Cardita. Brug. M. 



Equivalved, regular, in- 2 unequal cardinal Ligament exterior. (1) 

 equilateral. Fig. 41. teeth, one short and 



. straight under the beaks, 

 the other oblique^and 

 marginal. 



«t. Shell sub-cordiform or oval, more transverse than longitudinal. 

 . . more longitudinal than transverse. 



DIVISION VIII. ARCACE7E. 



GENERA. 



1. Cucui.LiEA. Lam. M. 



Equivalved, inequilate- Hinge linear, straight, 



ral, trapeziform, ventri- with little transverse 



cose; beaks wide apart, teeth; the extremities 



separated by the area of with from 2 to 5 ribs, 

 the ligament. Fig, 47. 



(2) 



M. 



2. Arc A. Lin. 



Sub-equivalved, inequi- Hinge in a straight 

 lateral ; beaks wide apart, line, with numerous pa- 

 separated by the area of rallel teeth ; no ribs to 

 the ligament : shell fre- the sides, 

 quently gaping. Fig. 44. 



of. Upper edge not crenated within. 

 j8. . . . . crenated within. 



Ligament exterior ; 

 muscular impression 

 forming a projecting an- 

 gular edge. 



Ligament exterior. (3) 



(1) The exterior side 4s very long, and the posterior very short : some are said 

 to attach themselves to marine bodies by threads, like the Mytili and Area. Foss. 

 in Great Oolite, Green Sand, Kelloway Stone, Bedford Limestone, Under Oolite, 

 Blue Lias, at Grignon, Tourahic, Sienna, Angers. 



(2) The greater number of the Arcacccp live under the sand, at a little distance 

 from the water's edge. The fossil species of the Cucullcea prove that it thickens 

 as it grows older, and the lateral rihs of the hinge are proportionally more nu- 

 merous; the area of the ligament also enlarges and acquires more furrows. Foss. 

 in Green Sand, at Bcauvais. 



(3) Several Area, are covered with a scaly or hairy epidermis, and some have 

 one valve larger than the other at the upper edge. One species is found at Timor 

 in a semi-fossil state. Foss. in Chalk Marl, Green Sand; at Houdan, Marl pits of 

 Touraine, Bourdeaux, Grignon, Parncs, Angers, Paris, Plaisancc, Turin, 



