400 MANUAL OF THE MOLLUSCA. 
Teredo, and Cardiwm, 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 Cardiwm retusum, L. Astarte 
excavata, and the genus Opis. Whena 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 Brachiopoda are opened. 
It consists of two parts, the hgament 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 
yalyes; 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 4 
large internal cartilage; in Arca 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 
- itacts asa hinge-muscle. (Pl. XXIII, Fig. 13.) 
The internal ligament, or cartilage, is lodged in furrows 
formed by the igamental plates, or in pits along the hinge- 
line; in Mya and Nucwa 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 Anatinide is furnished internally with a moyable 
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 eyen 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 Glycimeris 
* Only those technical terms which are used in a peculiar sense are here referred to; 
foi the rest, any Dictionary may be consulted, especially Roberts’s ‘ Etymologica] 
Dictionary of Geology,” published by Longman and Co, 
