MOLLUSCA: LAMELLIBRANCHIATA. 279 



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 Lamellibran- 

 chiata, 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 re- 

 gards the animal is always lateral, so that they are properly 

 termed "right" and "left" valves, instead of "ventral" and 

 "dorsal." 



The following are the chief points to be noticed in connec- 

 tion with the shell of any Lamellibranch : Each valve of the 

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

 which is turned more or less to one side ; so that more of the 

 shell is situated on one side of the apex than on the other. 

 The apex of the valve is called the " umbo," or " beak," and 

 is always turned towards the mouth of. the animal. Conse- 

 quently, the side of the shell towards which the umbones are 

 turned is the "anterior" side, and it is usually the shortest half 

 of the shell. The longer half of the shell, from which the 

 umbones turn away, is called the " posterior" side, but in some 

 cases this is equal to, or even shorter than, the anterior side. 

 The side of the shell where the beaks are situated, and where 

 the valves are united to one another, is called the "dorsal" 

 side, and the opposite margin, along which the shell opens, is 

 called the " ventral" side, or " base." The length of the shell 

 is measured from its anterior to its posterior margin, and its 

 breadth from the dorsal margin to the base. 



At the dorsal margin the valves are united to one another, 

 for a shorter or longer distance, along a line which is called 

 the "hinge-line." The union is effected in most shells by 

 means of a series of parts which interlock with one another 

 (the " teeth"), but these are sometimes absent, when the shell 

 is said to be "edentulous." Posterior to the umbones, in most 

 bivalves, is another structure passing between the valves, which 

 is called the " ligament," and which is usually composed of two 

 parts, either distinct or combined with one another. These 

 two parts are known as the " external ligament " (or the liga- 

 ment proper) and the "cartilage," and they constitute the 

 agency whereby the shell is opened, but one or other of them 

 may be absent. The ligament proper is outside the shell, and 

 consists of a band of horny fibres, passing from one valve to 

 the other just behind the beaks, in such a manner that it is put 

 upon the stretch when the shell is closed. The cartilage, or 



