MULTIVALVES AND BIVALVES. 



33 



on both sides of the beak. It may be wholly external or internal, 

 or partly one and partly the other; when it is external, the 

 edges of the shell are cut away to receive it, when it is internal, 

 it fits into a sort of pit known as the fossette. Really it is made up 

 of the ligament proper outside and the cartilage within, which are 

 generally continuous ; but in some cases, as in our example, which, 

 by the bye, is a species of Mactra, the cartilage is quite separate, 

 and is in a pit within the hinge. 



The hinge is the most important part of the shell, but, before we 

 consider it, let us put both valves together, so as to be clear with 

 regard to a few more terms. Looking down on the back of the 

 shell we see the beaks pointing forwards over the lunule ; the 

 anterior end is thus away from us, the posterior towards us. The 

 valve to our left when in this position is the left valve, the other 

 the right valve. The upper part of the shell, where the beaks are, is 

 the dorsal margin, opposite to it is the ventral margin, the other 

 margins being anterior and posterior, or fore and hind, as they are 

 often termed. The length of the shell is, in this book, taken as being 

 between the extreme points of the fore and hind margins; the 

 breadth being between the ventral and dorsal margins. Along the 

 dorsal margin is the hinge area. 



The hinge most frequently bears a series of prominences and 

 indentations interlocking with each other, and known as teeth. 

 Immediately beneath the beak are the hinge-teeth proper, the 

 " cardinals " (from cardo, the Latin for hinge), and on each side of 

 the cardinals, and extending some distance from them, are the 

 " laterals," the cardinals being, as a rule, compact and pointed, while 

 the laterals are narrow and thin. These teeth are so invariable in 

 their number and arrangement in different genera that they can be 

 used as guides in classification, and can be stated in the terms of a 

 dental formula, in which the middle term stands for the cardinal. 

 Thus i, 2, i, means that the hinge has two cardinal teeth with a 

 single lateral on each side of them. There is almost every variety 

 of hinge, from that with strong prominent teeth, to a mere series of 

 serrations all alike, and even to no teeth at all. 



One group of bivalves, the Pholadacea, have no teeth and no 

 hinge, and, at the most, but a very rudimentary ligament. They 

 are so different from the rest that we may as well sort them out 

 first. They can be recognised by the dorsal plates on the back of 

 the junction of the valves, and by the apophyses or falciform pro- 

 cesses within the shell to which some of the muscles are attached. In 

 British representatives of the sub-order the genera with long 

 apophyses are the borers Pholas and Pholadidea, the latter being 

 easily distinguished by the shell not being prickly all over, arid by 

 its having a sort of horny cup at its posterior end. 



The only native genus with short apophyses is Xylophaga. In 

 the same sub-order is now included the genus Teredo, of evil 

 reputation as a borer of wood. Its shell, which is difficult to 

 understand until it is seen in place on the animal, is continued into 

 a calcareous tube with long siphons, near the end of which, and 

 acting as valves, are a pair of pallets not unlike the scales of a 

 gigantic butterfly. No one is likely to mistake a Teredo for any 

 other mollusc. 



