CARDIACEA. 



727 



some malacologists, Tridacna is monomyary.) The shell is equi- 

 valve and thick, the hinge with or without teeth, the ligament exter- 

 nal, and the pallial line simple or slightly indented. This division 

 includes the two principal families of the Tridacnidce and Ca?'diidce. 



Family i. Tridacnidce. — In this family the shell is equivalve, 

 thick, not nacreous internally, and usually truncated or gaping in 

 front, the ventral margin being undulated or dentate, and the sur- 

 face ribbed. The hinge has a single cardinal tooth and one or two 

 posterior lateral teeth in each valve, and the ligament is external. 

 The foot is finger-like and usually secretes a byssus ; the mantle- 

 lobes are extensively united ; and the impressions of the adductors 

 are blended together and are subcentral in position. (x\ccording to 

 some authorities, the anterior adductor is really absent.) 



In the genus Tridacna itself (fig. 604) the shell is massive, of 

 large size, and subtrigonal in form ; and there is a wide byssal aper- 

 ture just in front of the beaks. On the other hand, in the closely 



Fig. 604. — Tridac7ia media. Tertiary. Interior of right valve. 



allied Hippopus the shell is closed, and the pedal aperture is only 

 indicated by small serrations of the margin of the valves. Tridacna 

 is only known in a fossil condition by a few species from the later 

 Tertiaries. The Palaeozoic Enrydesma has also been referred to this 

 family. 



Family 2. CardiidtE. — In this family are included the " Cockles," 

 in which the mantle is open in front to allow of the passage of a 

 large and sickle-shaped foot, while it is closed behind and gives 

 origin to two longer or shorter siphons. The shell is equivalve and 

 is not nacreous, usually heart-shaped in form, with radiating ribs, 

 and with the ventral margin toothed or wavy. The ligament is ex- 

 ternal, and the hinge carries one or two cardinal teeth in each valve, 

 usually with lateral teeth as well. There are two adductor-muscles, 

 and the pallial line is entire or slightly indented. 



In the genus Cardium are comprised the true Cockles, in which 

 the shell is ventricose, the beaks pronounced, and placed nearly in 



