138 ARCID^. 



addition to, or subtitution of, the ligament : hinge-plate fur- 

 nished with a row of close-set teeth in each valve, which 

 mutually interlock : muscular scars lateral, deep or distinctly 

 marked. 



The old and characteristic genus Area of Linne has 

 multiplied so fast since his time, that it now forms a 

 very numerous family. The quota which has been con- 

 tributed to it by palaeontology, or the record of extinct 

 races and generations, is nearly equal to that of recent 

 genera ; and in this sense the march of science may be 

 said to advance with almost the same rapidity in a re- 

 trograde as in a forward direction. Both zoologists 

 and palaeontologists are strenuous in the race ; but in- 

 stead of being hostile rivals, the only object of their 

 emulation is to assist each other and thus promote the 

 common cause. 



The character which makes this group (whether we re- 

 gard it as a genus, a family, or a set of families) so distinct 

 and self-contained is the peculiar structure of the hinge. 

 Instead of having, like other bivalves, only one, two, three, 

 or, at the most, four tubercular teeth under the beak, and 

 occasionally a single or double laminar fold on each side, 

 all the Arcidae are furnished with a symmetrical row 

 of these processes, occupying the whole of the hinge in 

 each valve, and interlocking like the real teeth of many 

 kinds of fish. This apparatus, aided by stout retractor 

 muscles and an elastic ligament, and in some genera by 

 a strong internal cartilage, enables the mollusk to keep 

 its house closed against most of its predatory enemies ; 

 and it is only when the shell is drilled by some canalifer- 

 ous Gasteropod, or else swallowed whole by a voracious 

 fish or by a member of the Bulla family, that its days 

 are numbered. Those species of Area which habitually 

 shelter themselves in the crevices of rocks, and are 



