CONCHIFEEA. 



401 



(PI. XXII., Figs. 13, 17), and in My a it is continued oyer the 

 siphons and closed mantle-lobes, making the shell appear 

 internal. 



The interior of bivalves is inscribed with characters borrowed 

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

 affinities thai), 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 present. In young shells 

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

 are often thickened, or even obliterated by irregular growth 

 [Hippojpodium) or the encroachment of the hinge-line [Fectun- 

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



The muscidar impressions are those of the adductors, the foot 

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



Fig, 210. Left valve. {Pecfenv r.ius.) Eight valve. 



a, a, adductor; p, pedal impression; m, palial line; Z, ligamental margin; c, c, ctr- 



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



The adductor impressions are usually simple, although the 



* The dentition of bivalve shells may be stated thus -.—cardinal teeth, 2.3 or | — 

 meaning 2 in the right valve, 3 in the left; lateral teeth 1 — ], 2.-»2, or 1 anterior and 1 

 posterior in the right valve, 2 anterior ar.d 2 posterior lat(ril teeth in the left valve. 



