252 MANUAL OF ZOOLOGY. 



they are c dorsal ' and 'ventral.' In the Lamellibranchiata, on 

 the other hand, the two valves are usually of nearly equal size 

 (equivalve), and are more developed on one side than on the 

 other (inequilateral) ; whilst their position as regards the ani- 

 mal is always lateral, so that they are properly termed ' right ' 

 and ' left ' valves, instead of ^ventral ' and ' dorsal.' 



The following are the chief points to be noticed in connection 

 with the shell of any .Lamellibranch. Each valve of the shell 

 may be regarded as essentially a hollow cone, the apex 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 

 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 um- 

 bones 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 mea- 

 sured from its anterior to its posterior margin, arid 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 pro- 

 cesses 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 



