median layer, d', inner layer; d", fibrous median layer ; d'", ex- 

 ternal layer. Magnified 104 diameters. 



13. Transverse section of the shell, but parallel with the fibres of the 



median layer. Magnified 104 diameters. 



14. Portion of the last, enlarged 317 diameters. 



15. Median layer of the same enlarged to 317 diameters, showing the 



granular aspect of another portion of the same specimen, where 

 the section has cut obliquely through one of the bundles of fibres. 



16. View of the median fibrous layer from above. Magnified 21 



diameters. 



