74 JOURNAL OF THE [April, 



The discussion of the structure of the diatom-valve turned, for 

 a long time, on the question whether the dotted markings were 

 caused by solid spherules or hemi spherules* of silex or by areolae 

 or alveoli in the shells. Elaborate rules were formulated by which 

 it was supposed the examination should be so conducted as to 

 decide indisputably when a dotted appearance was caused by the 

 one or the other structure. The study of the fractured edges of 

 broken shells, aided and illustrated by photography, may be said 

 to have settled this question in favor of the alveolar structure 

 some years ago. This done, we were prepared for the next ques- 

 tion, which was whether these alveoli are capped by a very thin 

 silicified film on one or on both sides of the shell. Are they or 

 are they not cellules in the shell, completely enclosed by a silici- 

 fied membrane ? Without asserting that this is definitively settled, 

 it may fairly be said that the prevalent opinion now is that the 

 alveoli are thus completely enclosed, but that the extremely thin 

 membranous caps are not so solidly silicified but that endos- 

 mose acts through them and the plant is nourished in this way. 



The next step in the discussion is the inquiry whether the 

 alveoli are interior cavities in a single homogeneous membrane, 

 or is the shell formed of two or more membranes. In the more 

 robust kinds it was demonstrated that there were at least two 

 plates or membranes in the shell. The larger Coscinodisci were 

 often found with the inner film, containing the so-called " eye- 

 spots," separated from the outer part of the shell; this inner film 

 being much thinner than the outer, and nearly flat, the shallow 

 depressions at the eye-spots being surrounded by a hexagonal 

 tracing which marked the place where the walls of the compara- 

 tively deep cellules had been attached. The outer film or plate 

 consists, apparently, of stout hexagonal structure capped on the 

 outer side. In several species this outer side has a secondary 

 marking of finer dots within the larger hexagons. In Triceratium 

 favus the inner film is that which has the secondary marking, 

 consisting of very fine dotted lines radiating from a common 

 centre and continuous over the whole shell. There can be little 

 doubt that these finer, secondary dots are analogous to the larger 

 ones and are indications of a pitted surface. In these coarser 

 forms it is quite conceivable that the structure should consist of 

 three plates, viz., an outer and an inner plate, each nearly flat, 



