396 DIATOMACECE. 



But the evidence of which we are in possession would appear 

 greatly to preponderate in proof of the vegetable character 

 of the Diatomacece ; thus, putting aside their silicious consti- 

 tution, and the partitions in the cells : the filaments of the 

 Meloseirce do not differ in the least in their modes of develope- 

 ment and growth from those of the Vesiculiferce, nor indeed 

 do those of Fragilaria differ very essentially from the fila- 

 ments of the true Alga ; in their reproduction, likewise, so 

 far as this has been ascertained, the Diatomacea correspond 

 with the Desmidea, and perhaps with all the lower Algce. 



In support of their animal nature may be urged their 

 colour, which is different from that of all other undoubted 

 vegetable productions, and the silex of which they are prin- 

 cipally constituted. These arguments, however, are not con- 

 clusive, nor are they of as much weight as those adduced in 

 proof of their vegetability. It is probable that in this matter, 

 as in many other disputed points, the truth lies in the mean, 

 and that the Diatomacece are neither exclusively animal, nor 

 exclusively vegetable, but of a nature intermediate. 



Owing to the presence of silex, the most indestructible 

 of substances, the species of this family are frequently found 

 beautifully preserved in a fossil state, and often in amazing 

 quantities all over the world. In the arts, the powder which 

 they form has been employed for polishing. Many of the 

 fossil species are identical with those now found in a recent 

 etate. 



Section i. 

 Frond filamentous. 



A. Filaments cylindrical. 



66. MELOSEIRA Ag. 



Char. Filaments cylindrical, subulate. Cells divided in the 

 centre or bilocular, sometimes vesicular. Junction sur- 

 faces of the cells either rounded or truncate, plain or 

 striated. 



