688 MOLLUSCA. 



side or " base." The length of the shell is measured from its an- 

 terior to its posterior margin, and its breadth from the dorsal margin 

 to the base. 



At the dorsal margin the valves are united to one another, for a 

 shorter or longer distance, along a line which is called the " hinge- 

 line." The union is effected in most shells by means of a series of 

 parts which interlock with one another (the " teeth "), but these are 

 sometimes absent, when the shell is said to be " edentulous." Pos- 

 terior to the umbones, in most Bivalves, is another structure passing 

 between the valves, which is called the " ligament," and which is 

 usually composed of two parts, either distinct or combined with one 

 another. These two parts are known as the " external ligament " 

 (or the ligament proper) and the " cartilage," and they constitute 

 the agency whereby the shell is opened ; but one or other of them 

 may be absent. The ligament proper is outside the shell, and 

 consists of a band of horny fibres, passing from one valve to the 

 other just behind the beaks, in such a manner that it is put upon the 

 stretch when the shell is closed. The cartilage, or internal ligament, 

 is lodged between the hinge-lines of the two valves, generally in one 

 or more " pits," or in special processes of the shell. It consists of 

 elastic fibres placed perpendicularly between the surfaces by which 

 it is contained, so that they are necessarily shortened and compressed 

 when the valves are shut. To open the shell, therefore, it is simply 

 necessary for the animal to relax the muscles which are provided 

 for the closure of the valves, whereupon the elastic force of the 

 ligament and cartilage is sufficient of itself to open the shell. 



Generally the hinge-line is curved, but it is sometimes straight. 

 The beaks are mostly more or less closely contiguous ; but they may 

 be removed from one another to a greater or less distance, and in 

 some anomalous forms they are not near one another at all. In 

 the genus Area the two beaks are separated from one another by 

 an oval or lozenge-shaped flat space or area. When teeth are 

 present, they differ much in their form and arrangement. In many 

 forms (fig. 554) the teeth are divisible into three sets — one group, 

 of one or more teeth, placed immediately beneath the umbo, and 

 known as the " cardinal teeth " ; and two groups on either side of 

 the preceding, termed the "lateral teeth." Sometimes there may 

 be lateral teeth only ; sometimes the cardinal teeth alone are 

 present ; and in some cases (Areidce) there is a row of similar and 

 equal teeth. 



While the opening of the valves of the shell of a Lamellibranch 

 is effected by the elastic force of the ligament, the closure of the 

 valves is effected by the contraction of one or two powerful muscles, 

 which are known as the " adductor muscles." In the majority of 

 Bivalves — hence termed Dimyaria — there are two adductor muscles, 



