24 
RoPER^ on Biddulphia. 
work of tlie valve. Now_, in tlie three species included in the 
first section of my proposed arrangement of the genus^ the 
valves are all formed on the same plan^ and^ I think^ it is a 
safe deduction that in all of them the markings are depres- 
sions. When we examine the valves of the second section, 
we find them as in Triceratium varying from distinct hexa- 
gonal reticulations^ to transverse markings, so obscure, that 
they are almost as difficult to resolve as in valves of some 
species of Pleurosigma, but an examination of the valves of 
B. rhombus and B. radiata, which vary considerably in size 
and the distinctness of the markings, clearly proves that the 
appearance of dots is merely the result of the minute size of 
the hexagons. It appears to me, therefore, we have fair 
grounds for supposing that in the other species in which the 
markings are unresolvable, the ultimate structure would be 
found similar, could we apply sufficient optical power. 
Now, if we turn from B. radiata to B. reticidata and 
Triceratium favus J we have valves that, with the same struc- 
ture, show that the spaces Avithin the hexagons are certainly 
formed of thinner siliceous plates, than the darker parts 
which bound them, and in fact are depressions on the surface 
of the valve ; and, as it is contrary to all analogy that we 
should have in the same genus a total diff'erence of structure 
in the substance of the siliceous membrane of the frustules, 
I think we may safely conclude that in those species which 
exhibit merely diagonal lines of apparent dots, as in B. 
Baileyii and Triceratium striolatum, when of small size, the 
structure is of a similar character to that in the species in 
which it is more clearly defined; and I am inclined, therefore, 
to agree with Dr. Griffith in the opinion that in these forms, 
as well as in Pleurosigma, the markings are depressions, and 
not elevations, on the surface of the valve. 
