8 



of papillae which give the shell the appearance of being 

 covered with a velvety tissue. These papillae are all 

 directed backwards towards the apex of the shell. In 

 sections (Text-fig. 2) the periostracum appears to be 

 made up of layers lying obliquely to the surface. A 

 spine is simply a prolongation of one of these layers. 



The outer layer (a) of shell (Text-fig. 2, a) is marked 

 by the possession of more organic substance than the 

 other layers. The calcareous substance crystallises 

 irregularly as mentioned above, and the appearance can 

 be understood better perhaps from the photograph 

 (PI. I, fig. 6) than from any description. 



The middle layer, Text-fig. 2 (6), is, as we have seen 

 above, marked in longitudinal sections by parallel lines 

 running perpendicular to the surface of the shell. In 

 transverse sections, however, the appearance of this layer 



Fig. 2. 



is, as Tullberg remarked, just like the inner layer (c) in 

 longitudinal section. This structure will be understood 

 better when the remaining two layers (c and d) inside it 

 are examined in transverse section. It will then be seen 

 that the layer (d) is marked like the middle layer (b), and 

 thus both (b) and (d) seem built of parallel columns in 

 longitudinal sections, whilst they are marked by oblique 

 lines in transverse section. The layer (c) is apparently 

 built up of parallel columns in transverse section. The 

 reason for this strange appearance is that the three inner 

 layers (b, c, and d) are built up in exactly the same way 



