MANUAL OF ZOOLOGY. 



of which is turned more or less to one side; so that more of 

 the shell is situated on one side of the apex than on the other. 

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

 is almost always turned towards the mouth of the animal. 

 Consequently, 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. In some Bivalves, however, the beaks are 

 " reversed," and are turned towards the posterior side 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 

 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 perpendicu- 

 larly 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 liga- 

 ment 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 





