point outwards and backwards. The apophysis, present as 

 a small rvidinient in the larval shell, has r^ro'Vfn out into 

 the shell cavity, pushing the mantle invagination before 

 it; and in this very rapidly attained stage, is almost as 

 large ccmparatively as in the adult. Meanwhile, tlie ven- 

 tral edge of the valve has grown rapidly, and there have 

 appeared on the dorsal and ventral portions th.e two knobs, 

 upon v/hich the two valves swing in this and subsequent 

 stages, during the mechanical process of boring. During 

 these changes, the hinge teeth have disappeared, probably 

 by absorption. The valves which, during larval life, have 

 svmng at the hinge so as to open or close the shell cavity 

 on the ventral side, come to swing u]ion the knobs along 

 a median transverse axis vertical to the axis of the 

 hinge. The greater growth of the valves on their ventral 

 edges causes them to gape before and beliind for the pro- 

 trusion of the foot and siphons. 



Growth of the valves continues with great rapidity. 

 The left valve of a specimen 1 mm. long is shown in oblique 

 view in fig. 15. The chief features that have been intro- 

 duced are as follows: The point of greatest growth is on 

 the vontral edge. The line of growth, and hence the rows 



