TRANSACTIONS OF MICROSCOPICAL SOCIETY. 
DESCRIPTION OF PLATE II. 
Illustrative of Dr. Gregory's Paper, On a remarkable Group 
of Diatomaceous Forms." 
The figures here given do not represent all the forms which may prove 
to be referable to Navicula varians, as described in the ' Transactions,' 
but are intended to show several of the more striking types, as well as 
some of the many transition forms by which these appear to be connected 
together. 
Thus, starting from fig. 1, which, as well as fig. 14, has the aspect of 
Pinnularia ohlonga much shortened ; we find that in fig. 2 it becomes 
larger and broader, and in fig. 4, still larger and longer, retaining the 
broad obtuse apices, while it acquires something of the character of P. 
peregrina. In fig. 6, it is again shorter and broader ; but in figs. 3, 3 d, 5, 
6 &, and 7, we observed it becoming narrow, sometimes rhombic, with 
acute apices in 5 and 5 h, and a slight constriction in the others. In figs. 
8, 9, and 10, the form is more elongated, especially towards the extremi- 
ties ; but in 10 h and 11, it is again shortened, retaining however, the 
constriction, which in fig. 11 is so strong as to render it subcapitate. 
From fig. 11, figs. 12, 13, and 13 h, lead us to figs. 14, 15, which are, as 
already stated, closely allied to fig. 1. This group includes forms either 
identical with, or very similar to P. ohlonga (shortened), P. peregrina, 
and P. acuta. The resemblance to the latter species, however, in figs. 3, 
3 5, and 5 &, is only in shape, the striation being quite distinct. 
. From fig. 14, we pass by fig. 25, which is shorter and more oval, to fig. 
16 {Navicula scutelloides, Sm.), which is of a short, broad, oval form, and 
sometimes almost orbicular. I have some doubts whether this last be 
referable to N. varians, or even whether it be a Navicula at all : it is 
understood that some good observers regard it as a Cocconeis. But I have 
no doubt as to the propriety of classing figs. 14 and 25 together. 
From fig. 11, we pass, in another direction, to figs. 24, 23, and 22, 
which are all subcapitate or capitate ; figs. 24 h, 21, 19 b, and 19, lead us 
gently to figs. 18 and 18 h, and through these to fig. 17, which is more 
decidedly apiculate than 18 h. This last, fig. 18 h, appears to be identical 
with the form described by me in the Mull earth as Pinnularia exigua. 
From figs. 23 and 22, we are led, through fig. 20 to fig. 20 h, which 
seems to be a variety of N. inflata, fig. 20 c. The latter is so well marked 
a species, that it may be doubted whether it can be a form of N. varians. 
I would not assert that it is so, but if figs. 20 &, and 20, are to be regarded 
as varieties of N. inflata, it is evident that that species is susceptible of 
considerable modifications, and we have already traced the connection 
between figs. 20 and 20 h, and others, which seem undoubtedly to belong 
to -A^. varians. 
From fig. 17, we now pass to figs. 27, 28, and 28 h, which are very 
striking forms, some of which have been supposed to be referable to N. 
Semen. But the striation in these forms is remarkably strong and con- 
spicuous, while in N. Semen it is quite the contrary. Indeed, in Ehren- 
berg's Microgeologie, where the striation of many forms is well represented, 
even where it is not easily resolvable, N. Semen is figured without strias. 
From the last-named group, we proceed, by figs. 29, 29 h, to figs. 30, 
30 &, 30 c, and 30 d, in which the type of form of Pinnularia gracilis 
appears, more or less distinctly. But the conspicuous and somewhat 
coarse striation at once distinguish these forms from that delicate species. 
In fig. 33, this type is fully developed in all dimensions, producing a 
splendid form; while in figs. 31, 32, 32 &, and 32 c, we pass through 
various modifications to the form or type of Navicula rhyncocephala. 
Want of space prevents the introduction of larger specimens of this type. 
