55 
divide, and empty themselves of their contents, which coalesce, 
and form a globular sporangium ; this, in some genera, is smooth — 
in others, it is covered with long spinous projections ; this sporan- 
gium, according to Hoffmeister, is not a single cell, but is composed 
of a number of cells, each of which assumes the form of the 
parent frustule. 
The absence of silex, and the delicate nature of the frustule, 
prevents its preservation in any of the numerous fresh water 
deposits generally so rich in diatoms. One genus of desmids has, 
however, been supposed to have a fossil representative, viz., the 
so-called Xanthidium, so common in flint, (it, however, more 
resembles the sporangium of the genus Cosmarium than the recent 
forms of Xanthidium.) 'I'hese forms were supposed by Ehrenberg 
to be remains of desmids, but as flints usually contain remains 
of foraminifera and bryozoa, indicative of marine origin, and the 
desmidiae are, as I before remarked, found only in fresh water 
the theory that they belong to the genus of desmids known 
as Xanthidium must be given up. The Xanthidium-like bodies in 
flint, are perhaps the remains of the gemmules of some sponge-like 
body, originally forming the nucleus of the flint. 
We will now proceed to the description of those remarkable 
forms of plant life called diatomaceee. The non-filamentous species 
were considered by the early observers to belong to the infusoria — 
a term still retained by some authors, although the majority of 
forms described by them are not found in infusions. Ehrenberg 
has described some of them in his Infusionsthierchen ; he not 
only overlooked their affinities to the algae, but even thought he 
saw organs of locomotion and digestion, and called them polygas- 
trica, or many-storaached, and, I believe, he stUl persists in their 
animal nature. He asserts that he has been successful in feeding 
them with colouring matter, (carmine or indigo,) but as no other 
observ^er has succeeded in doing it, I am inclined to believe that he 
made a scientific use of his imagination, or, in other words, fancied 
he saw what he wished to see. He says the colouring material was 
imbibed through what he calls the central umbilicus ; in this he 
was decidedly mistaken, as there is no aperture through which any 
substance, fluid or solid, could pass ; the umbilicus, on the con- 
trary, is a thickening of the cell wall. The modern arrangement 
of the diatomaceae is as follow : — Class, cryptogamia ; sub-class. 
