H. MORLAND ON DIATOM STRUCTURE. 803 



tions further than to say that, so far as I have been able to judge, 

 my observations are fully in accord with theirs. They maintain 

 that the markings on the diatom valves are " perforations," but 

 that the structure differs in different forms ; for instance, that of 

 Coscinodiscus ocutus-iridis may be likened to an india-rubber door- 

 mat with hexagonal holes, laid upon and joined to a thin sheet of 

 rubber, with a small circular hole in this sheet opposite the centre 

 of each hexagonal cell in the door-mat ; a valve cut obliquely 

 shows this very distinctly, and that the round marks at the bottom 

 of the cells are holes is clearly proved by an examination of the 

 edges. I have a section of Coscinodiscus which shows these holes 

 very distinctly under a i inch objective ; they are comparatively 

 large, about TI i^ 7 inch in diameter. With respect to Trinacria 

 Regina, one of the most common forms in the " Cementstein," the 

 structure is much more simple, being nothing more than simple 

 perforations ; the edges of the cut valves in all directions show 

 this very plainly. But besides this there are little specks of iron 

 pyrites here and there in this " Cementstein ; " if in very small 

 quantity, and coating any part of the diatom valves, it will always 

 be found between the dots, never on them unless in larger quantity, 

 in which case it is found all over the surface ; in no case are the 

 dots covered with iron pyrites and the intervening spaces left free. 

 Very thin sections of Pijxidicula cruciata, on which the markings 

 are comparatively large, will sometimes show the framework of the 

 valve in separate pieces, but still held in position by the balsam. 



The word " framework " reminds me that that is how we ought 

 to regard the siliceous valves of the Diatomacea?. In my view the 

 whole purpose of these valves is simply to support the living 

 substance of the diatom. If the solid " dot " theory were correct, 

 it would follow that the outer and inner portions of the living 

 diatom are completely shut off from one another with no com- 

 munication whatever between them ; if, on the other hand, the 

 valves be perforated, there would then be a continuity between the 

 two parts, besides which there would also be economy of material 

 used in building up this framework or skeleton. For my part I 

 cannot see the object of dots, but the case is very different if we 

 find these markings are really perforations. - 



According to Dr. Van Heurck's " Texte " to his " Synopsis of 

 the Belgian Diatoms," in that part treating on the structure of 

 diatom valves, it would seem he implies (not actually stating it) 



