1884.] 



MICROSCOPICAL JOURNAL. 



107 



makin«f a catalo^je of all in which 

 the details are large enough to give 

 a well-defined outline to a broken 

 edge. In the progressive series of 

 fine markings we sooner or later 

 reach the point where the thinness of 

 a film causes it to be lost in the gen- 

 eral background of the field, or when 

 the prismatic edge of a fracture makes 

 dirt'raction enough to fringe it with 

 lines of color or of apparent shadow, 

 which make every cautious observer 

 hesitate to affirm whether the boun- 

 dary is in or beyond one of the striae. 

 The fringes move with tlie slightest 

 motion of the fine adjustment of the 

 microscope, and the interpretation of 

 what we see is more or less modified 

 by the preconceived theories of the 

 observer. I have intended to draw 

 my examples of facts from specimens 

 found clearly within this limit of 

 doubtful discrimination. I am my- 

 self satisfied that in the coarser speci- 

 mens of difterent species of Pleuro- 

 sigma careful illumination and accu- 

 rate adjustment of good lenses show 

 the same characteristics of structure 

 at broken edges of shells which I 

 have described in the larger and 

 bolder forms. In regard to this, 

 however, I admit there is room for 

 dispute. In the matter of the color- 

 test, on the other hand, the evidence 

 seems to me clear. If the objective 

 is well adjusted, and the median line 

 is brought into focus so that it appears 

 a greenish white line of nearlv the 

 same tint as the general field, the dots 

 which are near enough to it to be in 

 the same plane are found to have the 

 same color. In Pleui'osigma formo- 

 sum, P. balticum^ P. attenuatmn^ 

 and the varieties closely allied to 

 each, the reticulation seems to be 

 thickened upon the outer edges of 

 the lines, so as to leave a cup-like 

 depression in the interstices, which 

 is yet consistent with double lamina; 

 below. We have seen that in Eupo- 

 discus this thickening becomes so 

 great as to be quite opaque. In Att- 

 lacodiscus Oregon ianus and in A. 

 oriefjfah's it is sometimes foimd thick 



enough to give a decidedlv dark color 

 to the reticulation of the surface. In 

 media of higher refractive index than 

 balsam this becomes still more no- 

 ticeable. In the pleurosigmas I have 

 named I think a similar thickening of 

 the lines (much more delicate, but 

 real) has taken place, and that this 

 gives the strong cross-hatching which 

 marks them. In the varieties more 

 closely allied to P. ang?ilatum the 

 shell is smoother, and in some of 

 these the surface, with high magnifi- 

 cation, and both by transmitted light 

 and under the vertical illuminator, is 

 found to resemble very closely that of 

 the distinctly areolated forms which 

 have been described. 



In conclusion, I will notice briefly 

 a few of the less regularly marked 

 diatoms, but which still seem to me 

 to corroborate the view of their struc- 

 ture which I have maintained. 



In a group of species allied to Aa- 

 vlcula prcetexta^ Ehr., including A^. 

 Hennedyi^ N. Indica, JV. chavata^ 

 etc., the regular stria are confined to 

 narrow bands at the margin and along 

 the median line, the intermediate space 

 being either hyaline or mottled in vary- 

 ing degrees of distinctness. Specimens 

 which have this mottling most distinct 

 exhibit it as a system of rather large 

 but faint dots, arranged in lines con- 

 tinuous with the distinct striae at the 

 margin, etc., but the dots in these lines 

 are irregularly spaced as to tlistance. 

 Occasionally an individual is found in 

 which the dots are as sharply ilefined 

 as in any of the smooth naviculie, and 

 giving the proof that they are areoljae 

 by fracture and by color. Arranged 

 in a series, therefore, they show us 

 that the diminishing distinctness of 

 marking is due to the progressive 

 shallowness of the depressions in one 

 of the laminui of the valve, until from 

 faintest mottling the dots disappear 

 entirely, leaving the interior space 

 smooth and hyaline. 



The study of these last assists us in 

 understanding the marking oi Helio- 

 pelta. In this splendid shell we have, 

 first, an outer lamina or film, finelv 



