248 WALLICH, ON TRICERATIUM. 
readily measured, when seen in fragment and in profile; and 
which, a priori, indicates the presence throughout each cell of 
the membranous structure from which it is thus deposited. 
The floor of these cells is also minutely punctate, the puncta 
being arranged in quincunx.* The minute puncta only re- 
quire careful illumination and a power of 400 diameters to 
render them quite distinct. At first sight the double out- 
line visible in the hexagonal cells, as seen in certain positions, 
might be considered as due to refraction. But, on obtaining 
a fragment in an oblique position, the perspective view of 
the receding cells leaves no doubt of their true character. I 
would observe that similar cellulation appears to me to exist 
in other discoid forms, although too minute to be as readily 
interpreted. 
On a side view of the valve, below the outer series of cells, 
a single row of small rectangular markings is observable. 
The valve is slightly constricted around the margin. The 
horn-like processes are directed upwards, as in 7. favus, but 
do not project beyond the angles. The constant character of 
the outline of the valves in this species is remarkable, imas- 
much as it answers to the figure formed by describing an arc of 
a circle, with a radius equal to the magnified diameter of one 
side of the valve; taking off, on that are, the same radius, 
and describing a second and like are; and, lastly, making the 
point of intersection thus obtained the centre wherefrom to 
complete, with still the same radius, the third arc, or side of 
the figure required. 
The measurements are— 
Diameter of each side of valve, from ‘0047 to ‘0175; dia- 
meter of each cell ‘00034; cells 3$in°001; depth of hexagonal- 
cell-walls -00026; length of fimbriz :00046; breadth the 
same ; striation on connecting band 48 in ‘001. 
Although, m general character, this species is no doubt 
closely allied to 7. favus, the remarkable cell-structure and 
fimbriated border sufficiently distinguishes it. 
Mr. Brightwell, in the ‘ Journal of Microscopical Science’ 
for July, 1853, p. 249, describes a new species by the name 
of T. grande and to this a figure is appended which, in the 
end view, is very similar to T. fimbriatum. But T. grande 
has no border, whereas 7. comptum, also described and 
* This basal plate, when the valve is fractured, presents a remarkable 
and somewhat obscure feature, inasmuch as its line of fracture does not 
always correspond with that of the valve generally, and would thus appear 
in some measure distinct—a fact which I cannot help thinking has led to 
such lower plate in some of the discoid forms being looked upon as distinct 
species, 
