DIATOMS 41 
families aiid genera according to ‘‘ The Conspectus of the 
Diatomacez,’’ arranged by Prof. H. L. Smith, of Geneva, 
NY: 
While the botanical status of the Diatomacee has given 
rise to discussion, experts now generally regard them as an 
Order of the Cenfervoid Algz. They are found in both 
fossil and receit forms. Living species abound in both 
salt and fresh water ; and some occur in bogs, mosses and 
in damp earth. There are but slight differences between 
marine and freshwater diatoms, though peculiarities of both 
classes are persistent. 
The Diatomaceze are unicellular, and each individual 
Diatom is a delicate membranous sac which with its con- 
tents are enclosed in a silicious covering. This flinty sheil, 
frustule it 1s called, is in two parts, one often slightly 
larger than the other, fitting in part over it, and forming 
together a ininiature box of rare beauty. ‘The line of 
junction or raphe between the two parts of the delicate box- 
like frustule is protected by a band or hoop, and the whole 
is covered with an almost imperceptable gelatinous cover- 
ing. These silicious frustules are the prominent feature of 
the Diatomaceez, and with their slight attachment in some 
species, gave to Diatoms the older name of Brittle-worts. 
Skilled observers record that the frustules contain granular 
protoplasm, a nucleus—sometimes two of them—and a 
germinal dot from which minute threads have been seen to 
radiate, 
The frustules of Diatoms take many different forms, 
They are found in circular, rectangular, wedge-shaped, 
stellate, sigmoidal and navicular outline, besides endless 
other less regular shapes. Somefrustules are ten or twelve 
times the length of others; but the minuteness of all is 
cause for astonishment. <A frustule of average length does 
not measure the two hundreth part of an inch; and genera 
shorter than the medium length are by far the more numer- 
ous. Some years since, Moller, an expert microscopist, 
