ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 945 



gatherings from Wedel raarsLes, or those of Holland. Wc have 

 Prof. H. L. Smith's authority for regarding this diatom as identical 

 with C. suhtilis, and it is, at farthest remove, only a variety of that 

 species. No distinctly hexagonal areolation is seen here, but the 

 jiunctae are round, though often so closely set as to lead the eye very 

 j>ersiiasively to the illusion of taking thera for hexagons. Eemember- 

 ing Nachet's figiu-e demonstrating the liability to mistake on this 

 point, and using to the full the advantage our widest angled glasses have 

 in seizing upon the surface, we shall soon satisfy ourselves that we 

 have round areolae in a shell of silex showing a pinkish tint. The 

 light within the areola, when the outline is in sharp definition, is of 

 the general pale greenish colour of the field. Depress the tube, and 

 the dots become red spherules ; decentre the light from the condenser 

 a little, and they stand out like little balls. Among these valves I 

 have found very numerous examples in which the fracture evidently 

 follows the line of tlie areolfe. In one sj)ecimen a segment had been 

 broken out, one side of it bounded by a regular radial line from the 

 centre of the shell to the circumference. In it the next row of areolaB 

 was plainly separated from the broken part by a line of silex of 

 apjjieciable width, on the outer edge of which the little irregularities 

 and indentations of the fracture showed where the divisions between 

 the adjacent dots had been. In both the American and the European 

 diatoms I have also occasionally found the two laminre of the shells 

 of this species separated partly or wholly, as has been noted in the 

 larger species of Coscinodiscus, and in such cases the fracture of 

 the inner lamina through the ' eye-spot ' is even more demonstrably 

 apparent than in the perfect shell. 



The evidence from fracture of the valve and from the general 

 appearance under the vertical illuminator, therefore, justifies the con- 

 clusion that the truest view of this diatom by transmitted light is that 

 which we have when the objective is so adjusted that the punctae 

 appear to be sharply drawn circles in a film of pale pink colour, the 

 circles themselves having a greenish-white light. We may conse- 

 quently reject the red spherules in this case as the product of 

 diffraction and interference of light. Another bit of experimental 

 evidence on the subject is foimd in the way in which, on a slight 

 motion of the mirror, the light will flash along behind a diatom, 

 lighting up the ai'eolae as it passes, and making the comparative dark- 

 ness of the thicker part of the shell apparent in a telling way. Dr. 

 Greville refers to this in his description of Aulacodiscus orientalis* 

 as making it very evident that the areola) in the clathrate frame- 

 work of that beautiful diatom are really thin,' window-like spaces, 

 through which the light flashes. The eflect is not easily described in 

 words, but it will be recognized by all who have had much exi^erience 

 in studying diatoms under the Microscope. 



Another species of diatoms will aid us to carry our induction a 

 little furtlier. In either of the gatherings I have mentioned we may 

 readily find specimens of Podosira maculata (Hyalodiscus stell'ujer 

 Bailey), and these will be found of very varying degrees of fineness 



* Traus. Micr. Hoc. Loinl, xii. (ISUI) p. 12. 



