CONCHOLOGY., 
35. Arca. Shelltransverse, mostly equivalve inc- 
guilateral, with the beaks separated by a transverse 
area. Hinge in a straight line, with numerous 
teeth alternately closing together and gradually 
enlarging from the centre ; lateral teeth none. 
Includes Arca Now, fusca, perforans, 
36. Pectunculus, Shell mostly orbicular, equi- 
vaive, closed; the beaks close together but 
becoming gradually more remote by age. Hinge 
in a curved line, with numerous teeth alternately 
closing together, the central ones obliterated ; 
lateral teeth none. 
Includes Arca Glycymeris, pilosa, and 
nummaria. 
37. Cardium. Shell more or less heart-shaped, 
-equivalve, mostly ribbed longitudinally, with the 
beaks prominent and close together. Hinge with 
two approximate penetrating teeth, and on each 
side a remote lateral inserted tooth. 
Includes the whole of this family. 
38. Mytilus. Shell longitudinal, equivalve, 
generally attached by a byssus issuing from the 
middle; with the beaks acute, nearly terminal 
_and straight. Hinge with small teeth varying in 
number, and alternately closing together; lateral 
teeth none. Ligament marginal and elongated. 
Includes M. edulis, pellucidus. 
39. Modiola. Shell somewhat transverse, equi- 
valve, with the hinder side very short; beaks 
nearly terminal and placed at the shorter side. 
Hinge without teeth, 
