PARTS OF BIVALVE SHELLS. 97 



careous matter, forming a tube. This calcareous tube, in some 

 cases, is of considerable size and thickness. In the Clavigella 

 one valve, and in the Aspergillum both valves, are soldered to 

 this tube, which, in the latter, presents a peculiar modification 

 of its exposed extremity, which resembles the end of the spout 

 of a watering-pot. No two shells can present a greater con- 

 trast than do those of the Placuna and Aspergillvm ; yet the 

 organization of their respective constructors is essentially the 

 same. In a classification of shells, the calcareous tubes of the 

 Dentalium, Serpula, Aspergillum, Vermetus, &c., would be 

 associated in the same general group : but it needs only to 

 observe how these products of animals, belonging not only to 

 different classes, but to distinct primary divisions of the animal 

 kingdom, are arranged in the cabinets of collectors, to be con- 

 vinced that Conchology as a classificatory science, apart from 

 Malacology (the science of mollusks), no longer exists. 



With regard to the structure and physiological relations of 

 bivalve shells, it may be observed, first, that in all acephalous 

 mollusks which breathe by distinct iamellaled gills (branchiae), 

 Lamellibranckiata, one valve corresponds to the left, the 

 other to the right side of the animal ; but in the brachiopodous 

 bivalves, one valve is applied to the ventral, and the other to the 

 dorsal aspect of the animal. 



In all the lamellibranch bivalves which are free, the two 

 valves are symmetrical, and the shell is termed equivalve ; in 

 all those which adhere by one of their valves to foreign bodies, 

 this valve is deeper and larger than the unattached valve ; such 

 shells are termed inequivalve. Of those acephalans which are 

 attached to foreign bodies by means of a byssus, some, as the 

 Tridacna (fig. 103), are equivalve, and both valves are 

 notched, to form the hole for the passage of the byssus ; while 

 others, as the Pecten (fig. 91), and Avicula (fig. 95), are 

 inequivalve, the byssus passing through a groove in the right 

 valve. 



If the shell of the common cockle be examined, each valve 

 will be seen to be produced into a conical prominence bent 

 towards, and nearly meeting at, that part by which the valves 

 are joined together. These prominences are termed the umbones> 

 or sometimes beaks. The apex of the umbo corresponds to 

 the apex of the univalve shell, and is the point at which the 

 development of the bivalve commences. When the apex is 

 directed in the transverse plane of the shell, and so placed, 

 that a division of the shell in that plane through the apices 

 shall divide the valve into two equal parts* the shell is termed 

 equilateral; of this form the Pecten (fig. 91) is au example, 

 9 



