WALKER-ARNOTT, ON ARACHNOIDISCUS, ETC. 197 
eligible, had he been aware of them, still those names which 
he has given, that are accompanied by a clear diagnosis, have 
now the right of priority. 
In my remarks I mentioned that the figure of the F.V. of 
Pleurosigma lanceolatum could not belong to it ; I have since 
received a specimen of this through the kindness of Dr. D., 
and to me the striz appear horizontal, while those of P. lan- 
ceolatum (S.V.) are diagonal. As to what P. lanceolatum 
itself is, different observers are entitled to hold different 
opinions; Mr. Roper’s P. transversale (3 is, I believe, allowed 
by all to be identical; and if I do not coincide with him, it 
is solely because there seems to me more points of dissimi- 
larity from, than of resemblance to, P. transversale. Taking 
into consideration that P. estuarii is “frequently direct,” 
and that what Professor Smith would have called the “ type” 
of that species has the ends “ somewhat produced ” or api- 
culate, and also the striation, I am almost satisfied that 
P. lanceolatum is a form of it peculiar to clean sand; but 
as yet neither P. danceolatum* nor the apiculate state of 
P. estuarii have occurred, so far as I know, sufficiently 
isolated to allow of any positive deduction being drawn. 
What is considered the non-apiculate state of P. estuarii, 
has been got copiously in some gatherings; but that chiefly 
differs from a small form of P. angulatum by the slightly 
more difficult striz, and may be considered one of a nume- 
rous group of intermediate diatoms, none of which can be 
referred with certainty to any species as at present (too 
stringently) limited, and yet are destitute of any marked 
peculiarity to permit of a separation,—a group that will 
ultimately cause the union of several “ fest species’ of that 
genus. The P. lanceolatum of Dr. Donkin must not be 
confounded with Mr. Norman’s species of the same name, 
(noticed, but not defined, in the ‘Ann. of Nat. Hist.’ for 
1857, vol. xx, p. 159); this last does not seem to me distinct 
from small forms of P. strigosum, Sm.+ I referred P. 
* Thave the same form from Cumbrae in the Clyde, accompanied by Tozxo- 
nidea insignis and Gregoriana, and Pleur. angulatum. If P. lanceolutum be 
distinct from P. estuariz, it is to it that Zor. insignis may be referred; but I 
cannot indicate any marks by which the anomalous state of each is to be re- 
cognised, except by the normal form which accompanies it. Zor. Gregoriana 
I refer to the sand form of Pl. angulatum. 
+ I may here mention that “ P. s¢rigosum” seems to have been adopted 
by Smith on the supposition that the Hull Wav. strigosa, of Harrison, was 
the same; this, however, I have ascertained, is not the case. WJ. strigosa 
of Harrison and Sollitt (‘ Micr. Journ.,’ ii, p. 62), is P. angulatum, Sm.; 
while their V. angulata is P. quadratum, Sm. ; their N. lineata is P. elonga- 
tum, Sm. It is not, however, improbable that these and some others form 
a single species. 
