of Diatomaceous Forms, 
15 
much finer striae ; in fact, agreeing in this respect with the 
four species just named. To this group belong the form 
figured in my paper on the Mull deposit as Pinnularia exigua ; 
that figured in the same paper as a doubtful form of P. 
radiosa, or between that species and P. peregrina, and a 
considerable number of other forms, which not only have the 
striae inclined and otherwise arranged exactly as in N, varians, 
but pass into one another by intermediate forms. Their 
aspect is quite distinct from that of N. varians^ because the 
striae, being much finer, cease to be conspicuous, as they are so 
remarkably in N. varians. To this group belongs also a form 
I lately described * as iV] lafiuscula, and I am inclined to 
, believe that the group includes JV. semen, JV. rhgncocephala, 
P. gracilis, P. radiosa, and others, just as I suspect that the 
group of varians includes JV. peregrina and P. (qy N ?) 
ohlonga. In favour of this supposition, I may mention that a 
friend informs me that the striae of P. gracilis have been 
found by him to be moniliform, although the fact may not yet 
be thoroughly established. This, it will be observed, corres- 
ponds to Mr. Smith's observation on the striae of P. peregrina. 
It is well known that the striae in P. gracilis are somewhat 
obscure, and that in this, as well as in number, they differ from 
those of N. varians. But we can now see how it was that 
several of the forms of N. varians were referred to P. gracilis. 
The latter, with its normal striation, appears to belong to 
the second group which I have mentioned, and which, 
for convenience, may be called N. mutahilis. 
It will be observed that if, in the case of N. varians, the 
two characters of variableness of outline and variableness in 
the number of striae should be found united, N. varians and 
N. mutahilis would then constitute but one group, divided, in 
the first instance, into those with 16 strige in "001" or N. 
varians, and those with 26 striae in • 001," or N. mutahilis, 
whether these divisions be regarded as species, or as subgenera. 
I have only farther to add, at present, that both these 
groups are widely distributed and often abundant, whether in 
the shape of the known species, such as P. peregrina, P. 
gracilis, &c., or in that of the types now first pointed out. I 
have named those gatherings or deposits in which they occur 
most abundantly ; but there are few mixed fresh-water gather- 
ings in which some of them do not occur. I have it, 
fortunately, in my power to supply observers with some of the 
most interesting gatherings, and I shall be happy to forward 
small portions of these, or slides, where the material is very 
scanty, to such microscopists as may wish to examine them. 
* In the paper already alluded to in a preceding note. 
