86 



THE AMEEICAK MONTHLY 



[May, 



line the spot becomes quite deeply red. 

 If we next select a valve in which the 

 dots are a little more distant from 

 each other and evidently round (the 

 scheme of marking and the marginal 

 •spines being the same as in the larger 

 specimens), we shall find the same 

 conduct with regard to color when 

 the objective is lowered or raised ; 

 that the fracture indubitably follows 

 the line of the dots, and that under 

 the vertical illuminator the smaller 

 dry specimen is not distinguishable 

 from the larger except in the round- 

 ness of the areolae. 



Pass now from C subtilis^ as we 

 find it along our own coast in gather- 

 ings shown in Peticolas's slides from 

 Jacksonville or Fernandina, Fla., to 

 the Odontodiscus subtilis of Moller's 

 ,type plate or his slides of gatherings 

 from Wedel marshes or those of Hol- 

 land. We have Prof. H. L. Smith's 

 authority for regarding this diatom as 

 identical with C. subtilis^ and it is, 

 at farthest remove, only a variety of 

 that species. No distinctly hexagonal 

 areolation'is seen here, but the punc- 

 t£e are round, though often so closely 

 set as to lead the eye very persuasively 

 to the illusion of taking them for hex- 

 agons. Remembering Nachet's fig- 

 ure demonstrating the liability to 

 mistake on this point, and using to 

 the full the advantage our widest an- 

 gled 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 out- 

 line is in sharp definition, is of the 

 general pale greenish color of the 

 field. Depress the tube, and the dots 

 become red spherules ; decenter the 

 light from the condenser a little, and 

 they stand out like little balls. Among 

 these valves I have found very nu- 

 merous examples in which the frac- 

 ture evidently follows the line of the 

 areolae. In one specimen a segment 

 had been broken out, one side of it 

 bounded by a regular radial line from 

 the centre of the shell to the circum- 

 ference. In it the next row of areolae 



was plainly separated from the broken 

 part by a line of silex of appreciable 

 width, on the outer edge of which 

 the little irregularities and indenta- 

 tions of the fracture showed where 

 the divisions between the adjacent 

 dots had been. In both the Ameri- 

 can and the European diatoms I have 

 also occasionally found the two lam- 

 inae of the shells of this species sepa- 

 rated partly or wholly, as has been 

 noted in the larger species of Coscin- 

 odiscus^ and in such cases the frac- 

 ture 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 appear- 

 ance under the vertical illuminator, 

 therefore, justifies the conclusion 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 shai'ply- 

 drawn circles in a film of pale pink 

 color, the circles themselves having 

 a greenish white light. We may 

 consequently reject the red spherules, 

 in this case, as the product of diffrac- 

 tion and interfei'ence of light. An- 

 other bit of experimental evidence on 

 the subject is found in the way in 

 which, on a slight motion of the mir- 

 ror, the light will flash along behind 

 a diatom, lighting up the areolae as it 

 passes, and making the comparative 

 darkness of the thicker part of the 

 shell apparent in a telling way. Dr. 

 Greville refers to this in his descrip- 

 tion oi Aulacodiscus orientalis (^^. 

 M. 7., vol. IV, N. S. 12, Trans.), 

 as making it very evident that the 

 areolae in the clathrate frame-work of 

 that beautiful diatom are really thin, 

 window-like spaces, through which 

 the light flashes. The effect is not 

 easily described in words, but it will 

 be recognized by all who have had 

 much experience in studying diatoms 

 under the microscope. 



Another species of diatoms will aid 

 us to carry our induction a little fur- 

 ther. In either of the gatherings I 

 have mentioned we mav readily find 



