198 WALKER-ARNOTT, ON ARACHNOIDISCUS, ETC. 
Wansbeckii, Donk., to Amphiprora Ralfsii; the examination 
of a specimen from Dr. Donkin satisfies me that I was 
wrong, and that it is a true Pleurosigma, and the same 
which Smith has called P. Balticum 8; I learn from Mr. 
Roper that his opinion quite coincides with my own. This 
also is the same which is called Navicula scalprum by Kutzing, 
as far at least as regards the form found by De Brébisson 
(see ‘ Kutz. Sp. Alg.,’ p. 85); and it may be the same as 
that figured by Kutzing (Bac. tab. 30, fig. 13), from Trinidad, 
and since called by him N. Scalpellum, but it scarcely agrees 
with the figure of N. scalprum given by Gaillon and Turpin 
(who first gave the name), in the ‘ Mem. du Mus.,’ xv, t. 10, 
fig. 3, and which figure is copied by Kutzing (Bae. tab. 4, 
fig. 25), and appears rather to indicate P. Hippocampus, 
Sm. In P. Balticum, B and y of Smith, the strie are as 
numerous as 64 or 65 in ‘001; and from notes before me 
I find that some others raise that number to 85; this 
creates a doubt if these two varieties ought not to be 
separated from a species which has only about 38. I believe 
that the usual state of P. Balticum is scarcely or not at all 
found at Hull, whilst the var. y occurs copiously at Hessle, 
and sparingly in several other places in that neighbour- 
hood; this, however, by itself, affords no valid reason for 
the separation, any more than that the mixture of all the 
three would prove them only to be varieties of the same 
species. 
AmpuHiprora.—The verbal distinction between this genus 
and its cognates Navicula and Pleurosigma is so slight as 
to be easily passed over. It is unnecessary to refer to 
Kutzing’s generic character of Amphiprora, which is obtained 
from the Front View only, while that of Navicula (including 
Pleurosigma) is derived from the Side View; “the so-called 
wings (a/@), or projections (as has been remarked by Mene- 
ghini), belong to the secondary surfaces (S. V.) and consti- 
tute the only distinctive character of the Amphiprore.” B 
comparing Smith’s analytical table at p. 9 of vol. i. of his 
‘Synopsis of Br. Diat.,’ with the characters given at pp. 43, 46, 
and 61, it will be seen that his views are that in Amphiprora 
the F. V. of the frustule and valves is deeply constricted late- 
rally, while the valves, or S. V., are furnished at the median line 
with a ridge or keel; in the two others, on the other hand, 
the F. V. is without a conspicuous constriction, and the valve 
is plane or convex merely, and the median line destitute of 
a keel. There is thus a double character, and when the one 
is not very decided the other may be taken asa guide. I 
