58 BIVALVE SHELLS. 



muscles b}'- which the valves of the Brachiopoda are opened. It 

 consists of two parts, the ligament properly- so-called, and the 

 cartilage; they exist either combined or distinct, and sometimes 

 one is developed and not the other. The external ligament is a 

 horny substance, similar to the epidermis which clothes the 

 valves ; it is usually attached to ridges on the posterior hinge- 

 margins, behind the umbones, and is consequently stretched by 

 the closing of the valves. The ligament is large in the river- 

 mussels, and small in the Mactras and My as, which have a large 

 internal cartilage ; in Area and Pectunculus the ligament is 

 spread over a flat, lozenge-shaped area, situated between the 

 umbones, and furrowed with cartilage grooves. In Chama and 

 Isocardia the ligament splits in front, and forms a spiral round 

 each umbo. The Pholades have no ligament, but the anterior 

 adductor is shifted to such a position on the hinge-margin that 

 it acts as a hinge-muscle. 



The internal ligament, or cartilage, is lodged in furrows formed 

 by the ligamental plates, or in pits along the hinge-line ; in Mya 

 and Nucula it is contained in a spoon-shaped process of one or 

 both valves. It is composed of elastic fibres placed perpendicu- 

 larl}^ to the surfaces between which it is contained, and is slightly 

 iridescent when broken; it is compressed by the closing of- the 

 valves, and tends forcibly to open them as soon as the pressure 

 of the muscles is removed. The name Amphidesma (double 

 ligament) was given to certain bivalves, on the supposition that 

 the separation of the cartilage from the ligament was peculiar 

 to them. The cartilage-pit of many of the Anatinidae is fur- 

 nished internally with a movable 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 which is organically 

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

 remarkable extent in Solemya and Glycimeris, 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 

 afl^nities 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 (ii, 31'. 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 



