84 GREVILLE, ON DIATOMACES. 
in N. Lyra they are, according to Professor Smith, only 20 
in*001" (rather under the mark according to my estimate). 
2. The invariably converging points of the blank spaces. I 
may add, in addition, that there is a striking flatness in the 
value of N. forcipata ; and that the blank spaces are defined 
by hard, sharp lines of contour, caused apparently by an 
abrupt depression in the surface of the valve. This is so 
conspicuous, that, under a moderate magnifying power, the 
eye dwells rather on the two parallel dark lines than on the 
blank space they enclose. The general outline, also, of the 
valve is not variable, as in N. Lyra, being never, as far as I 
have seen, produced at the ends, but always presenting an 
uninterrupted symmetrical curve. The Northumberland 
examples are the finest which have come under my notice; 
those from the Clyde are, for the most part, much smaller, 
—many of them even minute,—thus exhibiting a range of 
size as extensive as in N. Lyra and its allies. 
7. Pinnularia semiplena, Grev. 
Valve linear-elliptical, subacute; cost radiate, distant, 
very short in the middle, and becoming gradually longer 
towards the extremities, leaving an elongate, lozenge-shaped, 
centrical, blank space; length ‘0024; breadth about ‘0006" ; 
coste 15 in ‘001", (Fig. 12.) 
Navicula angulosa, var. B. Greg. ‘Trans. Mic. Soc.,’ v. iv, 
p. 42, Plate V, fig. 8*, 
Marine. Glenshira sand, Professor Gregory. Lamlash 
Bay, dredged by Professor Balfour. 
Having had abundant opportunities of observing this form, 
especially in my recent examination of many of Professor 
Gregory’s slides, I feel quite convinced that it is distinct from 
Navicula angulosa. 1 have not seen any approach towards 
an intermediate state. Indeed, I do not know any species 
more constant in regard to size and every other character. 
8. Achnanthes gregoriana, Grev. 
Front view of frustule broadly linear; striz very fine ; 
length -0060" to 0080"; breadth -0010" to 0015". — (Figs. 
13, 14.) 
Marine. Lamlash Bay; dredged by Professor Balfour, 
1857. 
Although a considerable number of the scattered frustules 
of this species have occurred both to Professor Gregory and 
myself, no other view than the one represented in the plate 
has been observed. In point of size it rivals A. longipes, but 
