Bibliographical Notices, 411 



present work. There cannot be a doubt that the best, perhaps the 

 only certain characters, are obtainable from those parts, and we wish 

 that Mr. Hassall had universally followed the plan which he appears 

 to have laid down for himself ; but in several of the families the spe- 

 cific characters are almost wholly derived from the relative diameter 

 of the filaments. The size of the filaments would doubtless be a 

 valuable and most convenient mode of distinguishing the plants if it 

 could be described in such a manner as to be always determinable, 

 but comparative size can at no time be depended upon, unless the 

 object Avith which the comparison is made be previously known. To 

 show the absurdity of such comparative characters, and how totally 

 useless a considerable portion of Mr. Hassall's definitions of nume- 

 rous species becomes, we will take a single series of species of the 

 genus Zygnema. 



Species 41 of the genus Zygnema is Z. duhium (Hass.); it has the 

 "filaments rather more slender than those of Z. Jenneri," in which 

 they are slenderer than in Z, vesicatum, when they equal those of 

 Z. diductum, in it equalling Z. intermedium, and so on through twelve 

 species to Z. orbiculare (Hass.), in which their diameter and length 

 is " very considerable.'* This is not a selected instance, but the first 

 which occurred to us, and might be backed by numerous others. It 

 is true that this forms only a part of the character, but to our mind 

 the remainder is not much better defined, and, at any rate, these 

 useless expressions ought not to have occupied so conspicuous a 

 position. 



We have felt it our duty to point out the very great fault just no- 

 ticed, and must now, in justice to our author, call attention to the 

 excellent mode in which he has divided that same genus Zygnema 

 into sections, by the " truncated," not inverted, and '* inverted ex- 

 tremities of the cells :" even here the figures are not all quite cor- 

 rect ; the dissepiments of the separated and connected cells being 

 figured similarly — the latter quite correctly ; but when the cells are 

 separated the central part is always protruded, not inverted, as re- 

 presented on these plates. 



In many cases, such as in the genera Vaucheria, Draparnaldia,Rivu- 

 laria, &c., where the magnified figures seem well-deserving of praise, 

 the want of a small portion of the plant represented of the natural 

 size is very observable. 



Algologists will be much struck by the union of the long-known 

 Conferva glomerata and C. cegagropila under the name of Cladophora 

 glomerata. Mr. Hassall's theory of the formation of the latter is, that 

 " a specimen by the force of some mountain stream swollen by re- 

 cent rains becomes forced from its attachment ; as it is carried along 

 by the current, it is made to revolve upon itself, until at last a per- 

 fect ball is formed of it, which finally becomes deposited in some 

 basin or reservoir." We cannot deny that this may be a correct 

 view of the facts — indeed it may be doubted if any person is qualified 

 to do so ; but we may observe, that the very still lakes and pools 

 in which C. (jegagropi'la is usually found are not all of them supplied 

 by swift streams, and that therefore some doubts may be allowed. 



2G2 



