A. A. C. ELIOT MERLIN ON NAVICULA SMITEII AND N. CRABRO. 249 



in length, has been found on the Tkurn Naples circle slide, 

 apparently identical in size and form with the specimen depicted 

 by Van Heurck, Plate B, fig. 23. It is mounted inside out, 

 and shows the distinctive diagonal double rows of " pearls" well, 

 while focussing down through these to the under (really the 

 outer) surface, the secondary perforated structure can be in- 

 distinctly but unmistakably seen. That it should be visible at 

 all through the entire thickness of the valve proves that it 

 cannot be difficult, even in this small form, when properly viewed 

 from the upper side. It is therefore now quite certain that both 

 large and small examples of N. Smithii from the Gulf of Naples 

 have the primaries crowned with a very beautiful kind of 

 secondary structure, which should assuredly be within the reach 

 of any good oil-immersion lens capable of standing a large 

 working aperture. There is no question whatever of any 

 .exceptional keenness of vision being required in this instance, 

 and I feel convinced that many members of a Club such as ours 

 must be provided with an optical outfit quite sufficient for the 

 purpose. In good specimens of the larger kinds, a |-in. Powell 

 objective of N.A. '72, made in 1850, has been lately found to 

 .show the secondaries with critical axial illumination, while the 

 aprochromatic |-in. of N.A. 1'42 exhibits them on a valve 

 measuring only about '04 mm. in length. 



Navicula crabro Ehr. {X. pandura, Breb.). 



Further observations show that the N. crabro, a form analo- 

 gous to, although differently shaped from N. Smithii, and 

 possessing, like the latter, a double row of " pearls," also exhibits 

 an exactly similar secondary structure. When the primaries are 

 in high " white dot " focus each is found to be composed of a 

 group of minute white points, but with a lower (and truer) focus 

 these clearly appear as the " black dot" images of perforations 



