L AMELLIBRANCHI ATA. 467 



near one another at all. In the Arcadce 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. 322) 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; sometimes the cardinal teeth alone are 

 present ; and in some cases (Arcadce) there is a row of simi- 

 lar and equal teeth. 



In the interior of the shell of the Bivalves are found cer- 

 tain markings which are often of great importance to the 

 palaeontologist. The body is enclosed in an expansion of 

 the dorsal integument, which constitutes the " mantle " or 

 " pallium," whereby the shell is secreted. Towards its cir- 

 cumference 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. 323, a). 

 In some of the Bivalves the two halves or " lobes " of the 

 mantle are united at their margins, so that the animal is 

 enveloped in an almost closed sac. In these cases it is 

 necessary that there should be orifices in the mantle-sac by 

 which .water can be admitted to the gills, and can be expelled 

 again from the body. The margins or lips of these orifices 

 are usually drawn out or extended into longer or shorter 

 muscular tubes, which are termed the " siphons," and which 

 may be either separate, or may be united to one another 

 along one side. The Bivalves which possess these siphons 

 are said to be " siphonate," and there are two leading modi- 

 fications in the arrangement of these tubes. In the Sipho- 

 nate Bivalves which spend their existence buried in sand or 

 mud, as well as in many other cases, the siphons are long, 

 and can be partially or entirely retracted within the shell by 

 means of special muscles, called the " retractor muscles of 

 the siphons." In these cases, the pallial line does not run 

 in an unbroken curve, but is deflected inwards posteriorly, 

 so as to form an indentation or bay, which is termed the 



