362 FRESH-WATER ALG^. [Biatomacea , 



by the coupling of the cells, and union of their contents/' 

 This description is taken from an admirable book by 

 Mr. Ralfs, of Penzance, on British Desmidiea?, — a work of 

 great research, and illustrated by many beautiful plates. I 

 shall not attempt to lead my young fi'iends into the depths 

 of microscopic Algology, yet, in touching at all on fresh- 

 water Algse, I could not refrain from giving a slight notice 

 of this exceedingly interesting tribe. They are all inha- 

 bitants of fresh water. Their colour is green, with the ex- 

 ception of a few of one genus, whose outer integument is 

 coloured, though the internal matter is green, wliile their 

 most obvious peculiarities are the beautiful variety of their 

 forms, and their external markings; their most distinguishing 

 characteristic, as Mr. Kalf observes, is the evident division 

 into two valves, or segments. Each cell, or joint, of the 

 JDesmidiea consists of two similar valves or segments, and 

 the line of junction is in general well marked. Mr. Ealfs 

 u decidedly of opinion that the two valves are but one 

 cell, differing on this point from any other writers on the 

 subject, except Professor Kiitzing, who in his ^Phycologia 

 Germanica ' has arrived at the same conclusion, by inde- 

 pendent observations. Mr. Ralfs states that the multipli- 

 cation of the cells by repeated transverse divisions is full of 



