38 



The remaining part of the prismatic layer stuck fast to the mother-of- 

 pearl of the crest. On this latter part, the crest, on the side of the pris- 

 matic layer, shows itself as a line. If we now look at ground pieces of 

 this crest, taken perpendicularly on its linear direction, it first strikes 

 us that the crest is quite massive and consists of layers of lime and thin 

 lamellae of periostracum, alternately. This number of lamellae varies 

 much according to our making a cross ground piece in different places 

 of the crest. I counted 1 — 5 of them, without any regularity in their 

 arrangement or appearance. Only one lamella is to be found constantly, 

 namely one that runs exactly in the middle of the crest. This lamella 

 originates from a layer of periostracum, which is lying betAveen the 

 prismatic layers of the shell. I clearly saw it bending almost perpendi- 

 cularly at the basis of the crest and farther running on in a parallel 



Fig. 1. AìiDUOììta cyyìiea. * 5. 



way with the usual layers of the shell. This lamella, the principal la- 

 mella, as it might be called, seems to me to have arisen first, and after- 

 wards the other lamellae might have been formed by the scaling of the 

 principal lamella, while lime was liberally formed everywhere between. 

 So all this together has formed the crest rising almost perpendicularly 

 on the shell of Anodonta. How and by what has this crest come about 

 now? We know nothing about it with certainty. Ai the very time 

 when I was occupied with these researches (May 1912), a study from 

 Dr. A. Rub bel was published, on these and similar abnormalities, in 

 the Zool. Anz. 14. 6. 1912, Volume 39, Nr. 21/22 page 632—643. 

 Rubbel too saw these crests and thinks that they have been caused by 

 "Fremdkörper". Once Rubbel even found a case of two crests above 

 each other of which he gives a picture. I have never found this latter 

 case. The first, inner crest of these two lying above each other, which 

 Rubbel represents is the same as my case, and Rubbel too mentions 

 a lamella of periostracum, running between the two crests. By a lucky 



