ZOOLOGICAL AND BOTANICAL ASSOCIATION. 97 



be no doubt correctly, esteemed to belong to the vegetable world, but 

 into the discussion it is not the province of this slight sketch to enter. 



It is now my duty to revert to the kindred order, Desmidiaceao. 

 Whatever doubt may have existed as to the true nature of the Diatoma- 

 ceae, I am somewhat at a loss to see how there could have been any he- 

 sitation with regard to the vegetable nature of the Desmidiaceae. In the 

 highest and most recent authority we have on these organisms, Halfs* 

 elegant monograph, — they are thus defined: — " Fresh- water figured 

 mucous and microscopic Algae, of a green colour ; transverse division 

 mostly complete, but in some genera incomplete ; cells or joints of two 

 symmetrical halves, the junction always marked by a division of the 

 endochrome, often also by a constriction. Sporangia formed by the 

 coupling of the cells and union of their contents." These are microscopic 

 gelatinous organisms, of a green colour, growing in fresh water only, 

 the cellulose walls of which are covered with " pearly granules," or 

 minute puncta, or asperities, or are smooth, without any siliceous coat, 

 of peculiar and varied forms, such as oval, lunate, cylindrical, triangular, 

 or compressed, &c, usually with variously formed rays or lobes, or quite 

 simple, presenting a bilateral symmetry ; the junction of the halves 

 marked by a division or interruption of the green contents, often, though 

 not always, by a constriction of the frond itself; the opposite valves con- 

 nected by a suture ; the individual cells either free, arranged in linear 

 series, forming a filament, collected into faggot-like bundles, or dis- 

 posed in circular, flattened, star-like groups. It will be seen that the 

 most striking points of distinction from Diatomaceae are the absence of 

 the siliceous figured coat, and the contents being green, not brownish or 

 yellow. The Desmidiaceae are very striking objects under the microscope, 

 from the singularity and variety of their forms, and their curious external 

 appendages and lobes. That which is most distinctive and characteristic 

 in their appearance is their bilateral symmetry, usually, though not 

 always, accompanied by a more or less deep constriction between the 

 segments of which each cell is composed. But in these respects there 

 exists all shades of difference in the various genera. For example, in 

 the genus Scenedesmus the symmetrical outline is wanting ; in Pedias- 

 trum it is indicated by an external notch on the outside only ; in Clos- 

 terium and Penium, as well as in a new form I have the honour in this 

 communication to bring forward, there is no constriction (nevertheless 

 the tendency to the bilateral division is indicated in those forms by the 



ZOOL. A BOT. SOC. PBOC. — VOL. I. 



