WALLICH, ON TRICERATIUM. 243 
those figures become landmarks, and th-ough them are 
afforded the means of ready comparison and reclassification. 
In no family, perhaps, is the remodelling of characters, and, 
in this particular ease, even of generic nam2, more necessary 
than in Triceratium. Recent additions to it clearly showing 
that outline, the character upon which it was originally, and, 
I may say, almost entirely, instituted by Ehrenberg, is not to 
be relied on; and that mere form may vary to a very great 
extent, whilst other characters, derived from important struc- 
tural analogies, at once point out how little value is, in reality, 
to be attached to it. Two of the species I am now about to 
describe exhibit this circumstance in a remarkable degree; 
the one a normally four-sided, the other a normally five- sided 
form; but both, nevertheless, distinct Triceraiia. 
The first of these, to which I propose giving the specific 
name of 7’. serratum, was obtained by me at St. Helena, 
along with numerous other new and highly interesting forms 
of living Diatoms, in dredging at oon twenty to thirty 
fathoms. Its characters are as follows 
Frustule free, constituting a ped prism. Valves 
quadrilateral, quadrangular—furnished with a_horn-like 
process at each angle ; and from four to six elongated spines 
furcate at their extremities. Connecting band composed of 
four quadrangular plates, joined together “by regularly ‘ dove- 
tailed”’ margins. These plates, in common with the valves 
_themselves, “marked with a delicate but well-defined hexa- 
gonal cellulation. 
This form is remarkable chiefly for the very peculiar struc- 
ture of its connecting membrane, which exhibits four distinct 
plates, having their communicating margins serrated, so as 
to fit into each other with accuracy. From this character the 
name is borrowed. The notches, or serre, are rectangular. 
Across each plate, during division, there is to be seen an 
arcuate narrow band, along which the cellulation is inter- 
rupted. ‘This band is expanded at its extremities. As divi- 
sion advances, each plate may be observed to consist of fwo 
layers, on the concave aspect of the arcuate band; which 
recede from each other; the upper one exhibiting the nor- 
mal cellulation ; whereas the lower (which is, in reality, a 
continuation of the other half of this plate) presents only a 
number of dots, the cellulation being imperfectly developed. 
In a memoir by Mr. Brightwell, “of Norwich, to which I 
shall again have occasion to refer, he mentions that the 
siliceous plates, forming the connecting membrane of the 
Triceratia generally, “are formed of several distinct layers 
of silex, dividing like the thin divisions of talc.” These 
