If.x 



X. — Biddulphia Mobiliensis. 

 By Edward M. Nelson. 



(Read February 19, 1908.) 



This diatom may be popularly described as being of the well- 

 known Isthmia type, and consequently much like a pocket cigar- 

 case. Probably a diatomist would say that the Isthmia was a 

 Biddulphia, but as this note is written for microscopists in general, 

 and not for diatomists only, it will be better to describe this 

 Biddulphia as being like an Isthmia, a common microscopical 

 object. 



Upon the side of this diatom strise, which count 41,000 and 

 32,000 per inch (1,610 and 1,260 per mm.), can be seen with a 

 low power, but with any lens of moderate aperture the diatom 

 can easily be dotted. 



When the object is examined under the most critical condi- 

 tions, with a very large axial solid cone of illumination, a suitable 

 blue-green screen, and a power of not less than 2,000 diameters, 

 the primary areolations will be found to contain a very minute 

 secondary structure. 



This structure is so delicate that it is not possible to hold the 

 image for long at a time. In general, four small dots will be 

 perceived in each primary, and if this had been all, it would have 

 been better not to mention the fact, because the image might 

 merely be a diffraction phenomenon ; but the investigation was 

 continued until some primaries which had five, and even six, 

 secondary dots in them had been found, thus proving that this 

 secondary structure is an entity. 



The diatom was sent to Mr. Merlin, who has kindly examined 

 it, and has confirmed the observation that all the primaries do not 

 have the same number of secondary dots. 



It is to be regretted that this note is not accompanied by even 

 a rough drawing of these secondaries. The image is excessively 

 difficult, and cannot be held long enough to draw ; the eye has 

 repeatedly to be rested in order to get even a momentary glimpse 

 of this tenuous structure. 



This is, so far as I know, the smallest primary in which any 

 secondary structure has been seen. There can be no doubt that 

 secondary structures which have been found to be present in so 

 many species of diatoms are of great importance to the organism, 



