412 EXTERNAL FORM OF SHELLS. 



subcentral posterior adductor muscle, were called by Lamarck 

 Monomyaires, the apex is generally in or near the centre ; 

 while in most of the other genera it is placed more or less 

 towards the anterior extremity of this margin, and is some- 

 times incurved. 



In some of these shells the apex is spirally twisted, and 

 the spire becomes more developed as they increase in size. 

 Now this could not take place if the valves remained inse- 

 parably united together at the same part of the dorsal 

 margin ; but it is provided for by the hinge of the shell being 

 gradually moved backwards on the edge of the valves, the 

 ligament separating in front of the hinge into two parts, one 

 of which diverges along each of the umbones, and forms a 

 spiral groove down the suture of the whorls. In Isocardia 

 the umbones seldom make more than half a turn, but in 

 one specimen of Chama in my collection they have made an 

 entire revolution, and in another a revolution and a half. 

 The valves of these shells being unequal, the spiral part of 

 the lower or attached valve is produced into an elongated 

 cone, while in the other it is depressed, and simply marked 

 with a spiral groove, like that of an operculum. 



In most bivalves the hinge-margin, which is deposited by 

 a part of the mantle extended behind and between the teeth, 

 increases in size much more slowly than the other margins 

 of the shell ; but in some free shells, such as the Arcae, this 

 part increases nearly as rapidly as the rest, and the umbones 

 thus become separated from each other by a lozenge-shaped 

 disk. In others which attach themselves to foreign bodies, 

 as the Spondyli and Ostreae, the hinge-margin of the attached 

 valve only enlarges, forming a triangular flat-topped process, 

 while that of the upper valve is scarcely increased in size. 

 Thus the cavity of the shell, as the growth proceeds, gradu- 

 ally retreats from the part by which the attachment first 

 took place. 



The direction followed by the whorls in passing down the 

 axis derives its origin from that which the shell takes in the 

 egg ; and is probably dependent upon the direction in which 

 the embryo rotated whilst enclosed therein. In most shells 

 they turn from left to right, and the mouth is on the right 

 side of the axis, when the shell is in its natural position ; 

 but in others, which are called sinistral or reversed, the 

 whorls are twisted in the contrary direction. The sinistral 

 direction appears to be constant in many species, especially 

 among the air-breathing Mollusca ; in all belonging to the 

 genus Clausilia, among the land ones ; and in all the Physae, 

 Planorbes and Ancyli, among those which inhabit fresh 



