CONCHIFERA. 401 
(Pl. XXII., Figs. 13, 17), and in Mya it is continued over 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 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 Arca, 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 Arca and Spondylus. The central teeth, those immediately 
beneath the wmbo, 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-defined ; in aged specimens they 
are often thickened, or even obliterated by irregular growth 
(Hippopodiwm) or the encroachment of the hinge-line (Pectun- 
culus), 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). 
— iy rc ir 
( Pecten v 7 at5.) Right valve. 
a, a, adductcr; p, pedal impression; m, palial line; J, tigamental margin; c, c, car- 
tilage ; é, 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 2— 
meaning 2 in the right valve, 3 in the /ef ; lateral teeth 1—), 2-42, or 1 anterior and 1 
posterior in the rzght valve, 2 anterior and 2 posterior lateral teeth in the /eft valve, 
