322 MANUAL OF ZOOLOGY. 



The apex of the valve is called the "umbo," or "beak," and 

 is always turned towards the mouth of the animal. Conse? 

 quently, the side of the shell towards which the umbones are 

 turned is the "anterior" side, and it is usually the shortest half 

 of the shell. The longer half of the shell, from which the 

 umbones turn away, is called the "posterior" side, but in some 

 cases this is equal to, or even shorter than, the anterior side. 

 The side of the shell where the beaks are situated, and where 

 the valves are united to one another, is called the "dorsal" 

 side ; and the opposite margin, along which the shell opens, is 

 called the " ventral" side, or "base." The length of the shell 

 is measured from its anterior 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." Posterior 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 liga- 

 ment 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 perpen- 

 dicularly 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 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 Arcadce. the two beaks are 

 separated from one another by an oval or lozenge-shaped 



