132 MOLLUSCA. LAMELLIBRANCHIATA. 



one occurs close to the anterior adductor, the other just 

 above the posterior adductor. Passing from one adductor 

 to the other in each valve, is a linear depression, the 

 pallial line (pi) ; this is caused by the attachment of the 

 muscles of the mantle to the shell. In some forms this 

 line runs evenly between the two adductor impressions 

 and parallel to the margin of the valve, it is then said to 

 be simple or entire. But in those genera which possess 

 retractile siphons, there is a bending in of the pallial line 

 just before reaching the posterior adductor, this inden- 

 tation is known as the pallial sinus (s), and is caused by 

 the muscles of the siphons. 



The hinge is formed by means of projections known as 

 teeth, which alternate in the two valves, the teeth of one 

 valve fitting into depressions between those of the other. 

 The teeth may be distinguished as cardinal and lateral. 

 The cardinal are those which occur under the umbones 

 (fig. 45 w, x, y) ; the lateral at the sides, those in front 

 being the anterior-lateral (z\ those behind, the posterior- 

 lateral. In some genera cardinal teeth only are found, in 

 others lateral or some of the lateral only. In a few 

 forms the two kinds of teeth cannot be separated, thus 

 Area possesses a row of numerous small teeth, all of 

 which are alike. Lastly some forms (e.g. Anodon) are 

 without teeth. The margin on which the teeth occur is 

 known as the hinge-line; generally it is curved, but in 

 some genera like Area it is straight. Sometimes (e.g. 

 Area) there is on each valve a flattened triangular space 

 between the hinge-line and the umbone, known as the 

 area ; when this is present the umbones of the two valves 

 are widely separated. 



In the brachiopods the valves are opened by the 

 divaricator muscles, but in the lamellibranchs the work of 



