as Dr. Gregory, on the Post-tertiary 
O. spinosa, which leads me to suppose that the markings are 
due to the septa having been removed and replaced by some 
new internal arrangement. 5. In both forms, the ends of the 
cylinders exhibit the spines, or appearance of spines, from 
which O. spinosa is named. It was for these reasons that I 
did not earlier mention O. mirahilis as a species ; and as for 
O. spinosa^ I had postponed it with other forms, otherwise 
both 'might long ago have been known. 
Let us now turn to the new forms. Here I must premise 
that a few of those new figures were described and figured in 
my former paper. I have figured these again, in some cases, be- 
cause the former figures were accidentally erroneous ; or in 
others, on account of additional peculiarities, or because I 
now understand the forms better than I was at that time able 
to do. By far the greater part of the forms now given are 
figured for the first time. 
1. Navicula rhombica^ n. sp. In my former paper are 
two figures of this species, which is very frequent in the 
sand. I now give two more figures, to complete the history 
of it. 
Length from 0*001" to 0-0025". Form rhombic, with 
somewhat acute apices as in the former figures, or elliptic 
lanceolate, with obtuse extremities, as in fig. 1. Striae fine, 
but easily seen with a good 1-4 or 1-5, about 45 in '001", but 
those near the middle of the valve much more distant, so as 
to be almost conspicuous ; the striae slightly inclined. Median 
line strong ; nodule large and well marked. Valve colourless, 
or pale yellow. 
The above characters sufficiently distinguish this species 
from N. rhomhoides, which, in the typical form, is always 
acutely rhombic, of a much darker colour, and has no definite 
central nodule, the two halves of the median line ending in 
sharp triangular points. The striae in N. rhomboides are so 
fine, that 1 have never yet been able to see them with a 1-5 
of extraordinary goodness, and they are indeed hardly to be 
resolved by the 1-8 ; they are also parallel. All these things 
unite to give to N. rhombica an aspect so entirely different 
from that of N, rhomboides^ that it is impossible to confound 
the two forms, where, as in the present deposit, they occur 
together. I may add that the variations of N. rhomboides, 
viz., N. crassinervia and JV. interrupta, W. G., are quite distinct 
from those of JV. rhombica. 1 state this, because some who 
have only seen the figures of JV. rhombica in my former paper, 
have supposed that it is only JV. rhomboides. Those who have 
seen the forms will admit that it is not possible for two species 
of the same genus to differ more thoroughly; but it is im- 
