CONCHIFERA. 24? 



when they cannot be completely shut. In Gaslrochana (PI. XXIII, fig. 15,) 

 the opening is anterior, and serves for the passage of the foot ; in My a it is 

 posterior and siphonal; in Solen and Glycimeris both ends are open. In 

 Bysso-arca (PL XVII. fig, 13,) there is a ventral opening formed by corres- 

 ponding notches in the margin of the valves, which serves for the passage of 

 the byssus ; in Pecten, Avicula, and Anomia, (fig. 176 s) the byssal notch 

 (or sinus) is confined to the right valve. 



The surface of bivalve shells is often ornamented with ribs which radiate 

 from the umbones to the margin, or with concentric ridges, which coincide 

 with the lines of growth. Sometimes the sculpturing is oblique, or wavy ; 

 in Tellina fabula it is confined to the right valve. In many species of 

 Pholas, Teredo and Cardium the surface is divided into two areas by a trans- 

 verse furrow, or by a change in the direction of the ribs. The lunule (see 

 fig. 14, p. 26,) is an oval space in front of the beaks; it is deeply im- 

 pressed in Cardium retusum, L. Astarte excavata and the genus Opts. 

 When a similar impression exists behind the beaks it is termed the escut- 

 cheon* 



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 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 Myas, 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. (PL 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 Nucula 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 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 carti- 



* Only those technical terms which are used in a. peculiar sense are here referred 

 to; for the rest, any Dictionary may be consulted, especially Roberta's Etymological 

 Dictionary of Geology, by Longman and Co. 



