28 



spat as indicated at e. Whether it is proper to regard 

 the superimposed series of individuals as generations 

 may be questioned, but as no more expressive word 

 occurs to me, I wish to be understood as using it here 

 with qualifications. 



The ''spat,'' upon careful examination of these beds, 

 we learn does not fix itself in any constant position ; in 

 some cases the young may have the hinge of the shell 

 directed downwards, upwards, or to the right or left 

 hand. Curiously enough the shells do not grow in the 

 directions in which the free edges of the valves are made 

 to assume in the young. Should the young happen to 

 be fixed hinge downwards the free edges of the valves 

 grow directly upwards; in case the hinge is directed 

 either to the right or to the left the layers of lime will 

 be deposited in such a way upon one side as to cause the 

 free edges of the valves to be eventually directed up- 

 wards, causing the umbonal portion of the valves to 

 describe an arc of 90^. In case the hinge is at first 

 directed upwards the layers of carbonate of lime will be 

 deposited in such a way by the mantle as to bring the 

 mouth of the shell upwards as shown in the individual a 

 Fig. 5. The attempt to get into a vertical position has 

 only been partially successful however, in this last case, 

 since the arc of 180^ seems a little too much to be tra- 

 versed in this way, even by the wonderful persistence of 

 effort of the inhabitant. This struggle to assume the 

 upright position is shown by the curved umbos and lines 

 of growth in other individuals, both old and young, as 

 indicated in the cases of h c, Fig 5. The habit of grow- 

 ing in the erect position, where the banks are prolific 

 and undisturbed causes the individuals to be very much 

 crowded together, so that they do not have a chance to 

 expand laterally. From this cause the individual oys- 

 ters become very narrow and greatly elongated ; the 

 peculiar forms which result are known as "cat's- 

 tongues" amongst oystermen, probably from a sugges- 



