34 INTRODUCTION. 



which are placed teeth, and pits to receive them on the two 

 valves reciprocally. Each of these it will be necessary to treat 

 of more at large ; observing, at the same time, that in some 

 species of Bivalves these parts may be wholly or partially 

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

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

 the hinge of which is destitute of teeth and ligament, the two 

 valves being kept together by loose cartilages, and by the con- 

 tracted space in which they are confined. 



Ligament 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 difference 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 with 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. 



