248 MANUAL OF THE MOLLUSCA. 



laa:e from the ligament was peculiar to them. The cartilage-pit of many of 

 the Anatinidtn is furnished internally with a moveable ossicle. 



The ligament is frequently preserved in fossil shells, such as the great 

 Cyprinas and Carditas of the London Clay, the Unios of the Wealden, and 

 even in some lower Silurian bivalves. 



All bivalves are clothed with an epidermis (v. p. 40) which is organi- 

 cally connected with the margin of the mantle. It is developed to a remark- 

 able extent in Solemya and Glycimeris (PL XXII. fig. 13, 17), and in My a 

 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 characterizes 

 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 pre- 

 sented 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-defined; in aged 

 specimens they are often thickened > or even obliterated by irregular growth 

 (Hippopodium) or the encroachment of the hinge-line (Pectunculus}. 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 p. 26.) 



The adductor impressions are usually simple, although the muscles 

 themselves may be composed of two elements,! as in Cytherea chione 

 (fig. 14, p. 26) and the common oyster. The impression of the posterior 

 adductor in Spondylus is double (PL XVI. fig. 15). In Pecten varius 

 (fig. 173, a, a,} large independent impressions are formed by the two por- 

 tions of the adductor, and in the left valve there is a third impression (p) 

 produced by the foot, which in the byssiferous pectens is a simple conical 

 muscle with a broad base. 



* The dentition of bivalve shells may be stated thus : cardinal teeth, 2.3 or f 

 meaning 2 in the right valve, 3 in the left; lateral teeth 11, 22, or 1 anterior 

 and 1 posterior in the right valve, 2 anterior and 2 posterior lateral teeth in the left 

 valve. 



t Compare the shell of modiola, PI. XVII. fig. 5, with the woodcut, fig. 177. 



