WALKER-ARNOTT, ON MARINE DIATOMS. 177 
might have been intended; this, which has two striz at the 
central nodule, much stronger and more distinct than the 
others, had certainly not been seen by Smith on the piece 
of mica he had received, if there at all. When Smith got 
the specimen from Saltcoats, he found the F.V. to accord 
with what he was led to think N. pectinalis, and as the valve 
was linear-elliptical, he has so described it in the 2d vol. of 
the ‘ Synopsis,’ p. 92; but his N. pectinalis was unknown to 
De Brébisson; it is a rare species, and as yet known to me 
only from the coast of Ayrshire and island of Cumbrae. 
Unfortunately he has there conjoined with it another form 
which I first found in Ayrshire (but soon after in Arran, 
and I have since got it from the west coast of England) ; this 
is much smaller, with broadly linear truncate valves, and 
radiating central strie. De Brébisson, in 1854, published 
his N. retusa, a species with linear valves, and which I feel 
certain, as already stated, was the one which was sent to 
Smith, in 1852, as N. pectinalis of De Brébisson ; this is very 
common in the mouth of the Clyde, and has been sent me 
from several other places considerably distant from each 
other ; itis adopted by Smith in his ‘ Synopsis.’ There results 
from this, if my views be correct, that N. pectinalis, Sm., is 
not that to which De Brébisson first gave the name; 
and that the one intended by De Brébisson is probably that 
which he afterwards published as N. retusa. The other two 
species (viz., the small one confounded with N. pectinalis by 
Smith, and the one at one time called Amphora ? quadrata by 
De Brébisson) are as yet undescribed ; the latter was first, in 
this country, found on the Northumberland coast by Dr. 
Donkin, and afterwards copiously at Tynemouth by the Rev. 
R. Taylor; I have since got it from Cumbrae in the Clyde, 
and I have seen it from Teignmouth in S. Devon, but very 
sparingly in both these localities. To this group also belongs 
Pinnularia rostellata, Greg. (the F.V. can scarcely be dis- 
tinguished from that of Nav. retusa) ; I do not see how this 
differs from Nav. apiculata, De Bréb. 
Synedra pulchella and minutissima are said to grow in fresh 
water, S. gracilis and acicularis m brackish. I find no 
difference in that respect ; all when in fresh water are got in the 
mouth of streams ; I never met with any of these at a consider- 
able elevation, nor, indeed, beyond the occasional if not con- 
stant rise of the tide ; between these four reputed species I find 
no certain marks of distinction. 8S. fasciculata is said by 
Smith to grow only in fresh water; while Agardh, 
Kiitzing, Greville, and others make it marine; both parties 
are correct, because they do not mean the same species. 
