42 DIATOMS 
mounted on a single slide 450 individual Diatoms. The 
circular space containing them was within the field of view 
of a two inch lens; that is, less in diameter than the third 
of an English inch. And in proof of his skill and patience, 
Moller grouped them ail in families, conformably with a 
printed explanatory list sent out with them. 
Much attention has been given to the surface markings 
on frustules of the Diatomaceee. They are described as 
striz, dots, depressions, hemispherical elevations, and 
letter-like signs. Many species show such markings 
symmetrically arranged ; aud when seen _ rightly 
illuminated, they are exquisitely beautiful. Diatoms of 
various families have striated frustules wlth a space 
between the striz less than the twenty thousandth of an 
inch. One species, the Amphipleura Pellucida, has long 
been noted for stri2 of special fineness. The lines on its 
frustules are so close together that on the more delicate 
- specimens one hundred lines measure but the thousandth 
part of an inch. That is said to be, with the best aid 
science can give, near the minimum visible ; for these lines 
are equivalent in closeness together to the closest ruled 
lines on a Nobert test plate distinctly seen, though Nobert 
is said to have ruled four or five finer bands never yet 
resolved. The Grammataphora Marina, a species found 
in both the Atlantic and Pacific Oceans, is noted for carry- 
ing on each frustule fine striz, and four characters like the 
letter gamma of the Greek alphabet. There are of this 
genus only thirteen species, four of which are British and 
nine are American. But where all are beautiful, selection 
ecomes invidious, and as difficult as it was for Sindbad to 
select diamonds in the land ef fairie. So of the discous 
aud other beautiful genera of Diatoms I must omit 
mention. 
Students of the Diatomacee formerly worked with 
microscopes of moderate power, that could just show lines 
on the Pleurosigma formosum, and similar frustules.- And 
