CONCHTFEEA. 



401 



(PI. XXII., Figs. 13, 11), and in Mi/a it is continued over the 

 siphons and closed mantle-lobes, making the shell appear 

 inter nciL 



The interior of biyalves is inscribed with characters borrowed 

 directly from the shell-fish, and affording a surer clue to its 

 afiBnities than those which the exterior presents. The structure 

 of the hinge characterises both families and genera, whilst the 

 condition of the respiratory and locomotive organs may be to 

 some extent inferred from the muscular markings. 



The margin of the shell on which the ligament and teeth 

 are situated, is termed the hinge-line. It is very long and 

 straight in Avicula and Area, very short in Vulsella, and curved 

 in most genera. The locomotive bivalves have generally the 

 strongest hinges, but the most perfect examples are presented 

 by Area and Spondylus. The central teeth, those immediately 

 beneath the umbo, are called hinge (or cardinal) teeth; those 

 on each side are lateral teeth. Sometimes lateral teeth are 

 developed, and not cardinal teeth {Alasmodon ; Kellia) : more 

 frequently the hinge-teeth alone are j)resent. In young shells 

 the teeth are sharp and well-defined ; in aged specimens they 

 are often thickened, or even obliterated by irregular growth 

 {Hippopodium) or the encroachment of the hinge-line [Pectiui- 

 adus). Many of the fixed and boring shells are edentulous.* 



The muscular impressions are those of the adductors, the foot 

 and byssus, the siphons, and the mantle (see pp. 19, 20). 



rig. 210. Left valve. { Peel en v >; itzs.) Eight valve. 



a, a, adductcr ; p, pedal impression ; m, pal ial liae ; ?, I gamental margin ; c, c, car- 

 tilage ; e, e, anterior ears ; b, byssal sinus. 



The adductor impressions are usually simple, although the 



* Tlie dentition of bivalve shells may be stated tlms :— cardinal teetli, 2.3 or § — 

 meaning 2 in the rigid valve, 3 in the left ; lateral teeth 1 — ), 2— .2, or 1 anterior and 1 

 VOsterior in the right valve, 2 antcrio;- ard 2 posterior lateral teeth in the IcJ-t valve. 



