2l6 MOLLUSCA. 



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 beak, 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 liga- 

 ment 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 near one another at all. 

 In the Arcada 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 some forms (fig. 167) 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 ; some- 

 times the cardinal teeth alone are present ; and in some cases 

 (Arcadce) there is a row of similar and equal teeth. 



In the interior of the shell of the Bivalves are found certain 

 markings which are often of great importance to the palaeon- 

 tologist. The body is enclosed in an expansion of the dorsal 

 integument, which constitutes the "mantle" or "pallium," 

 whereby the shell is secreted. Towards its circumference the 

 mantle is more or less completely united to the shell, leaving 

 in its interior, when the soft parts are removed, a more or less 

 distinctly impressed line, which is called the " pallial line " or 

 " pallial impression " (fig. 168, a). In some of the Bivalves the 

 two halves or " lobes " of the mantle are united at their mar- 

 gins, so that the animal is enveloped in an almost closed sac. 



