2 INTRODUCTION. 



different and conflicting views, every fact relating to their 

 history has undergone a more rigorous examination. 



The Desmidiese are of an herbaceous green colour ; a few 

 only of the Closteria have the integument coloured, but in all 

 the internal matter is green. 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, in my 

 opinion, belong to the Diatomacese. 



Their most obvious peculiarities are the beauty and variety 

 of their forms and their external markings and appendages ; 

 but their most distinctive character is the evident division 

 into two valves or segments. 



Each cell or joint in the Desmidie?e consists of two sym- 

 metrical valves or segments, and the suture or line of junc- 

 tion is in general well-marked ; in a few instances however, 

 as in Scenedesmus, it is determined principally by analogy. 

 In Pediastrum its situation is shown by a more or less evident 

 notch on the outer side, but no separation has been noticed. 

 The structures of Scenedesmus and Pediastrum are in fact 

 less known than those of the rest of the family ; and of their 

 modes of reproduction we are altogether ignorant. In the 

 other genera the suture eventually opens and allows the 

 escape of the contents, and it is indicated by either a trans- 

 verse line or a pale band, and usually also by a constric- 

 tion. 



An uninterrupted gradation may be traced from species in 

 which these characters are inconspicuous to those in which 

 they are fully developed : thus in Closterium and some species 

 of Penium there is no constriction ; in Tetmemorus, in some 

 Cosmaria, and in Hyalotheca, it is quite evident, although 

 still but slight ; in Didymoprium and Desmidium it is denoted 

 by a notch at each angle ; but in Spheerozosma, Micrasterias 

 and some other genera, the constriction is very deep, and the 

 connecting portion forms a mere chord between the segments, 



