ANATOMY OF THE LAMELLIBRANCHIATA. 125 



the beak in the lower valve. The situation of the ligament 

 is various ; in the Pecten it unites the ears of the valves ; in 

 the Area it is stretched over the wide space between the 

 beaks ; it unites the edges of the valves anterior to the beaks, 

 or is spread over the hmule in many other Dimyaria. When 

 as in the Bucardium, &c, the cartilage is external, and con- 

 vex and prominent above, its compression does not happen 

 from the pressure of the valves, as is the case with the species 

 with internal cartilages, but from the bending of it upon itself. 

 The elastic substance of the cartilage of this conformation 

 differs from that of the Pecten, &c, by its containing a por- 

 tion of carbonate of lime in its composition. 



The hinge is likewise commonly furnished with teeth, often, 

 as in the Trigonia, of most regular conformation ; developed 

 for the purpose of preventing the sliding of the valves upon 

 each other ; fitting between their fellows of the opposite side 

 with great harmony. The teeth are wanting or weak when 

 there is great strength of muscle or cartilage ; when the irre- 

 gularity of the edges of the valves prevents sliding motion ; 

 when the shell is small, flat and polished, and hence little 

 exposed to violence ; or when the hinge and cartilage are 

 long. They are, however, very numerous in the long hinge 

 of the Arcacea, compensating for the weakness of the carti- 

 lage. 



From the superior and posterior situation of the cartilage 

 in many bivalves, the anterior and inferior part of the shell 

 opens widest when the ligament acts, and from this part the 

 foot commonly protrudes. When the foot protrudes inferi- 

 ority, the cartilage is in the middle of the dorsal edge. 



In those genera which have gaping shells and long fleshy 

 syphons, the cartilage is internal and situated on a projecting 

 process of one of the valves ; by such a disposition the shell 

 is not readily quite closed nor much opened. The shell is 

 only allowed to be opened widely when the lobes of the man- 

 tle unite to a small extent ; as is done by the internal carti- 

 lage of many of the Monomyaria. 



When the foot is of a compressed form, from the position 

 of the ligament and cartilage, one on each side the beaks, 

 much motion is not provided for. When, as in the Area, the 

 foot is thick, we see in the linear hinge and in the remoteness 

 of the beaks, a provision for the considerable opening of the 

 valves by that organ, and in some species the valves them- 

 selves are gaping inferiorly for its exsertion. Besides the 

 teeth, the Osteodesma has a loose calcareous piece at the 

 hinge, before the internal ligament. In the Pholades there 



Vol. III.— No. 27. n. s. o 



