MOLLUSCA: LAMELLIBRANCHIATA. 397 



guished from that of a Brachiopod, irrespective of the great 

 difference in the structure of the animal in each. The shell 

 in the Brachiopoda, as we have seen, is rarely or never quite 

 equivalve, and always has its two sides equally developed 

 (equilateral) ; whilst the valves are placed antero-posteriorly 

 as regards the animal, one in front and one behind, so that 

 they are " dorsal " and " ventral." In the Lamellibranchiata, 

 on the other hand, the two valves are usually of nearly equal 

 size (equivalve), and are more developed on one side than on 

 the other (inequilateral) ; whilst their position as regards the 

 animal is always lateral, so that they are properly termed 

 " right " and " left " valves, instead of " ventral " and " dorsal." 

 It is to be remembered, however, that many of the Bivalves, 

 such as the Oysters, habitually lie on one side, in which case 



C 



Fig. 207. Left valve of Cytherea cMone. (After Woodward.) A, Anterior margin. 

 B, Posterior margin. C, Ventral margin or base ; -u Umbo ; h Ligament ; / Lunule ; 

 c Cardinal tooth; 1 1 Lateral teeth; a Anterior adductor; a' Posterior adductor; 

 p Pallial line ; * Pallial sinus, caused by the retractor muscles of the siphons. 



the valves, though really right and left, are called " upper " and 

 " lower." It is to be borne in mind also, that the two valves, 

 especially in the attached Bivalves, may be very unsymmetrical, 

 one valve being much larger or deeper than the other. Lastly, 

 there are some cases (e.g., Pectunculus) in which the shell be- 

 comes very nearly equilateral, the line drawn from the beaks 

 to the base dividing the shell into two almost equal halves.^ 



The following are the chief points to be noticed in connection 

 with the shell of any Lamellibranch (fig. 207) : Each valve of 

 the shell may be regarded as essentially a hollow cone, the apex 



