56 



HARDWICKE'S SCIENCE-GOSSIP. 



[March 1, 1870. 



small, elliptical, very convex, and it has one large 

 process nearly central. Erom this process to the 

 margin it is marked by fine striae, which cannot 

 easily be explained by figure (fig. 55). 



Fig. 55. Diatom from Guano, x 350. 



The study of the minute markings of some 

 diatoms has proved of essential service to the pro- 

 gress of microscopy. Diatoms have afforded tests 

 for the best methods of illumination by transmitted 

 light, and the various accounts of their structure 

 that have been recorded, and afterwards have been 

 proved to be erroneous, teach valuable lessons of 

 the necessity for caution in the interpretation of 

 appearauces. 



In the number of the Monthly Microscopical 

 Journal for December, it is more than hinted that 

 Aulacodisctis orientalis of Dr. Greville is but one of 

 the plates of A. oregamis. This statement suggests 

 the necessity for a careful re-examination of many 

 of the disk forms of diatoms. 



I have been accustomed to select diatoms with 

 the aid of a simple microscope, and a glass of about 

 an inch focal length, and very often I have taken 

 Aulacodisctis scaler, and turned it on its side, so 

 as to give the front view, and then I have brought 

 down the needle, with which I select the diatoms, 

 on the broad hoop, so as to separate the two valves, 

 that I might have both side by side to compare 

 their structure. In the front view, one valve 

 seemed to be convex and the other quite flat, and I 

 am led to doubt the constancy of the apiculi, which 

 have been considered characteristic features in this 

 species of diatom. 



Again, as Mr. Kitton has observed in the Micro- 

 scopical Journal for January, 1868, " the absence of 

 the margin is an accident of frequent occurrence." 

 This can be observed in the case of Arachnoidiscus 

 ornatus, for it is in a single case only out of very 

 many that the margin will be perfect, and the im- 

 perfect specimens could only be remarked by com- 

 parison with a complete specimen that may some- 

 times be found. In this diatom, also, interesting 

 although unimportant variations and imperfections 

 may frequently be observed in the umbilicus. A 

 great number of specimens may be obtained from 

 some samples of Ichaboe guano, and from them an 

 opportunity may be found for examining the dif- 



ferent plates of the diatom, and the peculiarities of 

 their structure. 



In an example of Navicula lyra which I possess, 

 there is a singular contortion in the markings of the 

 upper valve, and a difference of structure in the 

 valves themselves, which is very apparent and very 



a b 



Fie:. 56. Navicula lyra, x 350. 

 a, Upper valve. 0, Lower valve. 



remarkable. In fig. 56 a there is a view of this 

 diatom as seen with the upper value in focus, Eig. 

 56 b gives the appearance when the microscope is 

 focussed down to show the lower valve. The 

 power which resolves the lines of the lower valve 

 into clearly-marked and distinct dots, exhibits the 

 lines of the upper valve as coarse stria? which 

 cannot be thus resolved, and had the two valves 

 been separate, few would suppose them to be 

 portions of the same diatom. 



From the numerous instances of similar variations 

 that 1 have observed, I am led to agree with those 

 who apprehend that the possibility and extent of 

 variations of structure in the same species, and 

 even on the valves of the same diatom, have not 

 been sufficiently appreciated, and that if they had 

 been duly estimated, there would have been much 

 less of that extensive multiplication of supposed 

 new species on insufficient grounds, which is still 

 the reproach that can justly be cast upon the results 

 of the microscopical research of the Diatomacese. 



There is every reason to expect that in the course 

 of time the errors of past interpretation will be 

 gradually corrected, and now that the Royal Micro- 

 scopical Society is established as a national institu- 

 tion, microscopists scattered throughout the king- 

 dom will look to this society for the influences ne- 



