400 'MANUAL OP THE MOLLtTSCA. 



Teredo^ and Cardium, the surface is divided into two areas by 

 a transverse furrow, or by a change in the direction of the ribs. 

 The lunule (see Fig. 14, p. 20) is an oval space in. front of the 

 beaks ; it is deeply impressed in Cardium retusum, L. Astarte 

 excavata, and the^ genus Ojns. When a similar impression exists 

 behind the beaks, it is termed the escutcheon.* 



The ligament of the Conchifera forms a substitute for the 

 muscles by which the valves of the BracMopoda are opened. 

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

 the cartilage ; they exist either combined or distinct, and some* 

 times 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 Myas, which have £i 

 large internal cartilage ; in Area and Fectunculus the ligament 

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

 ambones, 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. (PI. XXIII., Fig. 13.) 



The internal ligament, or cartilage^ is lodged in furrows 

 formed by the ligamental plates, or in pits along the hinge- 

 line; in Mya and Nucida it is contained in a spoon-shaped 

 process of one or both valves. It is composed of elastic fibres 

 placed perpendicularly to the surfaces between which it is 

 contained, and is slightly iridescent when broken ; it is com- 

 pressed 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 Anatinidce is furnished 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 {v. p. 33) which 

 is organically connected with the margin of the mantle. It is 

 developed to a remarkable extent in Solemya and Olycimeris 



* Onh' those technical terms which are used in a peculiar sense are here refen-ed to; 

 foi the rest, any Dictionarj' may be consulted, especially Roberts's " Etj-mological 

 Dictionarj' of Geology " published by Longman and Co. 



