56 
Roper, on Triceratium Arcticum. 
Smith we find it included amongst the free forms of his first 
sub-tribe, though diff'ering widely in general structure from 
all the other genera included in it. 
The only statement that has been published, tending to 
throw light on the mode of growth of the larger species of 
Triceratium, is in the introduction to Mr. BrightwelFs valu- 
able synopsis of the genus, published in 1853, in ' Micr. Jour.,^ 
vol. i, p. 246, where, in alluding to a species named by him 
Arcticum'' (from its having been brought home by Dr. 
Sutherland from Beechy Island, in the Arctic seas), he 
says, The frustules of this species were found in a mass, 
unmixed with any other species of Diatomacese ; many of the 
perfect frustules have the endochrome in them, and when 
examined as first received, had very much the appearance of 
being attached to a small alga found with them.^^ Professor 
Smith had evidently entertained the idea that this might 
generally be the case ; as in the ' Synopsis,^ vol. i, which 
appeared before Mr. BrightwelPs paper was published, he 
says, " The frustules are probably at first attached to larger 
algse, but I have been unable to determine this point, from 
the isolated specimens which have fallen under my notice.^^"^ 
Since Mr. BrightwelFs paper, no further advance has been 
made in our knowledge of the mode of growth of the well- 
determined species of Triceratium, though the same author, 
in describing some gatherings made by Colonel Baddeley, has 
figured two peculiar forms under the names of T. undulatum 
and T. malleus, which were found in a living state and in 
filaments. t It will, however, require a more extended exami- 
nation of the structure and habits of this group, before they 
can be decidedly admitted into the present genus. 
The general structure of the valves in Triceratium, the 
horn-like processes at the angles, the frequent occurrence of 
spines on the surface, and the strongly sihceous and frequently 
reticulated connecting membrane, are so similar to that in 
Biddulphia, that I had long entertained the opinion that they 
were closely allied to that genus ; and any one referring to the 
figures of the perfect frustules given by Mr. Brightwell in his 
*^ Synopsis^ of the genus, especially in '^Mic. Jour.,^ vol. i, 
t. iv, fig. 5 a, and ^ Mic. Jour.,' vol. iv, t. xvii, fig. 11 6 and fig. 
9 or who has examined with a moderate power those of T. 
striolatum in the Thames mud, will see at once how closely 
they approximate on the F.V. to the frustules of Biddulphia. 
From a careful consideration of these structural peculiarities, 
* ' Smith's Syn./ vol. i, p. 26. 
f ' Mic. Jour./ vol. vi. p. 153. 
