24: 



NOTE ON NAVICULA SMITH U AND N. CRABRO. 



By A. A. 0. Eliot Merlin, F.R.M.S. 



{Read October \Wi, 1907.) 



While pleasantly occupied in examining the new and interesting 

 work on microscopy dedicated to our Club by its President, my 

 attention was attracted by the striking photograph of Xavicula 

 Smtihii (Plate X.), and the description annexed thereto aroused 

 my curiosity, and led me to hunt through my cabinet in search 

 of a specimen. 



Two valves, apparently exactly answering in shape and general 

 appearance to the species depicted in Plate IX. fig. 12 of Van 

 Heurck's Atlas of the Diatomaceae, but measuring "15 mm. in 

 length in place of *1 mm., were found on a Thum styrax circle 

 slide of forms from the Gulf of Naples. One of these happens 

 to be mounted so that the concave interior is exposed to view, 

 while the other exhibits the outer convex surface. 



On examining both under an apochromat of 1ST. A. 1*42, I was 

 surprised to find that the oblique double rows of " pearls " de- 

 scribed by Van Heurck (p. 91) as most essentially typical of the 

 species, appeared well defined on the specimen mounted inside 

 out, while the exterior convex surface of the other valve clearly 

 showed oblique double rows of closely adjacent, very distinct 

 primaries, capped and pierced with conspicuous secondary per- 

 forations. These secondaries, proving perfectly easy and obvious 

 under critical illumination with my Leitz long tube jVm. semi- 

 apochromat of 1ST. A. 1'32, should consequently be within the 

 grasp of any decently good, cheap, homogeneous immersion ob- 

 jective. Focussing down from the secondaries, it also proved easy 

 to see the lower orifices of the primaries in " black dot" focus; 

 these appeared sharp in outline, contracted, well separated, and 



