Bibliographical Notices. 41] 
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 with 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. dubium (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. egagropila 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. egagropila is usually found are not all of them supplied 
by swift streams, and that therefore some doubts may be allowed, 
2G2 
