466 LAMELLIBKANCHI ATA. 



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 meas- 

 ured 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 betwefen 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 liga- 

 ment " (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 be- 

 tween 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 short- 

 ened 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. 



The hinge -line is mostly curved, but it may be quite 

 straight. Generally the beaks are 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 



