Wallich, on the Diatom-valve. 145 
ness or coarseness of the markings^ is^ within given limits^ 
dependent on tlie conditions under which the sporangeal 
frustule gives egress to the germs of the new generation ; but 
that the ordinary process of division is, of itself, sufficient to 
bring about great variation, when operating through a vast 
succession of individuals."^ 
* In the discussion which followed the reading of Dr. Wallich's paper, 
Mr. Wenham stated that, with an object-glass of his own construction, 
having a focal distance of about -^th of an inch and a large aperture, he had 
ascertained beyond doubt, that in Pleurosigma angulatum^ and some others, 
the valves are composed wholly of spherical particles of silex, possessing high 
refractive properties. And he showed how all the various optical ap- 
pearances in the valves of the Diatomacese might be reconciled with the 
supposition that their structure was universally the same. — \Bds?\ 
