37 



stracum, which I described on page 577 and 578 of my first communi- 

 cation, is wanting. This is caused by the peculiar place of the damage, 

 viz. on the edge of the shell. Both the normal and the regenerated pe- 

 riostracum are almost on the same level here, and the regenerate does 

 not protrude, nor is it bubbly on the inside of the shell, as was the case 

 with the example described before see fig. 2 page 576), where the new 

 periostracum was lying quite in the depth in comparison with the nor- 

 mal periostracum of the shell. And this is the very reason why, in the 

 first case, the crust of lime has been well saved and in this case, where 

 it was quite on the surface of the shell, has been grazed off. 



On the inside of the shell I could clearly follow the cleft on the 

 mother-of-jDearl; the regenerate of the cleft had formed a crest on it. 

 On either side of this crest again were many very small pearls having 

 grown to it, and which, therefore, have to be considered as being con- 

 nected with the regenerate. Further, nowhere on the mother-of-pearl 

 were any pearls to be found. So this agrees with what I wrote before 

 in the middle of page 577. The •^ o figure of regenerated mussels, occur- 

 ring in nature, which I fixed at 1 in the first communication also appe- 

 ars to be right here. Besides I found in the same shell, near the top, 

 on the mother-of-pearl, on the inside of one of the shells a formation 

 which has nothing to do with regenerates, but which is remarkable 

 enough to be mentioned. There we see over a length of 2 cm and a 

 width of 0,4 cm a few little low crests running on the shell 2 longer 

 and 3 short ones, partly connected with each other. They look like 

 folds in the layer of mother-of-pearl and they owe their origin to the 

 circumstance that the mantle showed folds when the mother-of-pearl 

 was set off in this place. On the outside of the same shell nothing par- 

 ticular is to be discovered, and I am convinced that these folds have 

 nothing to do with regeneration. It is true that these folds are only 

 slightly developed, but it may occur that this development is very strong 

 and then the shell which shows such a development on the inside, gets 

 a very peculiar appearance. Among the hundred mussels mentioned 

 above I saw a magnificent example of this (fig. 1). There we see a crest 

 running over a length of 5 cm, which is 2 mm high in the middle, 

 and on the front and back side ends in a point and imperceptibly runs 

 into the mother-of-pearl. Above the crest are a number of very small 

 pearls, grown to it. This crest is almost perpendicular on the surface 

 of the shell and shows a slight curve in its course. On the outside of 

 the shell nothing remarkable is to be found. For further examination 

 I have sawed out of the middle of the shell a part with a piece of this 

 crest. The periostracum of this square piece let go and on the inside 

 this periostracum was still covered with a part of the prismatic layer. 



