Wallich^ on Siliceous Organisms, 49 
The history of the fossil Xanthidia of the Cretaceous flint 
nodules has hitherto been beset with a most perplexing diffi- 
culty. Apart from the much-vexed question as to the 
manner in which the siliceous element of the nodules 
became aggregated into masses^ so as to enshroud these struc- 
tures, the occurrence of organisms, held to be strictly in- 
habitants of fresh water, in deposits of unquestionable marine 
formation, presented an ample field for discussion and con- 
jecture. The Desmidiacese, of which family the Xanthidia 
constitute the sporangia, are thus described by Mr. llalfs : 
All the family are inhabitants of fresh water. Mr. Thwaites, 
indeed, has gathered two or three species in water slightly 
brackish, but the same species are also found in localities 
remote from the sea. Certain marine objects that have been 
classed with the Desmidiese have the internal matter of a 
brown colour, but these belong to the Diatomaceae.^^"^" 
In order, therefore, to account for their occurrence amongst 
purely marine deposits, it was deemed necessary to assume that 
they had been washed down by rivers or floods, or transported 
by winds from the land to the sea ; where, after gradually sub- 
siding, they became incorporated with the siliceous material 
in which they were found imbedded. The tough, semi-horny 
texture of their outer covering, and well-known power of 
withstanding extreme climatic changes, seemed to fayour 
this idea of their indestructibility, even under conditions so 
opposed to those in which they originally lived. But the 
explanation was, at best, unsatisfactory. 
The discovery of two well-marked varieties of Xanthidia, 
in a recent state, amongst the alimentary contents of the 
Salpm, was, therefore, fraught with no slight interest. From 
the condition of their endochrome it is certain they had but 
recently been inclosed in the cavities of these creatures ; and 
from the situation in which they were discovered, it is equally 
certain that they could not have been derived from fresh- 
water sources by any of the agencies above referred to. But 
whether or not these bodies are the sporangia of genuine pelagic 
Desmidiacece, they certainly exhibit an identity of appearance 
and structure with the fossil forms, too striking to admit of 
question. The probability, however, of their being true 
pelagic representatives of this family is greatly enhanced by 
the occurrence, in the same material, of bodies closely resem- 
bling the Closteria. These have only been met with sparingly, 
and it has been impossible, therefore, to determine their 
nature with certainty. But the strongest evidence in favour 
of their identity with the fossil Xanthidia is derived from 
* Ralfs' 'British Desmidiese.' Introd. p. 2. 
