34 INTRODUCTION. 



whicli are placed teeth^ and pits to receive them on the two 

 valves reciprocally. Each of these it will be necessary to ex- 

 plain more fully ; observing, at the same time, that in some 

 species of Bivalves these parts many be wholly or partially 

 wanting. Thus we meet with some shells, such as the Muscle, 

 without teeth ; and in the group containing Pholas, &c. the 

 hinge is destitute of teeth and ligament, the two valves being 

 kept together by loose cartilages, and by the stone in the 

 hollow of which they are confined. 



Liyament and Ligamentary Cartilage of the Hinge. 



These two distinct substances have been described by many 



writers, as though, composing the same mass, they were of 



one substance; but the diflference may very easily be explained. 



The true ligament is external, being fixed on the edge of one 



valve behind the umbones, and passing over in an arch to the 



corresponding edge of the other, very correctly retaining the 



name of ligament, because it serves the purpose of binding 



the two together. The thick, elastic substance, which Mr. 



Gray names the cartilage, is sometimes found in connexion 



with the ligament, so as to form one mass wit^i it, although 



it is always separable and placed within it : it is sometimes 



placed quite within the shell, and separated from the ligament, 



in a pit or hollow formed for its reception in the hinge lamina, 



near the centre. It is found in both valves, and being elastic, 



the portion in one valve presses against that in the other, so 



as to keep the valves apart, unless voluntarily closed by the 



adductor muscles of the animal. The ligament is sometimes 



spread over an external area, as in Area, while the cartilage 



is placed in several grooves of the same area, beneath the 



outer covering. 



