» RoPERj on Triceratium Arcticum. 57 
(altliough in the absence of any direct proof) ^ I stated in a 
paper read before this Society last year, that Triceratium, in 
any good natural arrangement of the genera, oughtto be placed 
as intermediate between Amphitetras and Biddulphia"^ — in 
fact, the more extended knowledge we now have of the curious 
four-sided frustules that occasionally occur, of which four 
have been recorded as established varieties of T. favus, arma- 
tum, formosum, and Arcticum, would tend to show a very 
close affinity to the former genus. 
The object of the present communication is, to show that Tri- 
ceratium has always been erroneously, considered a free form. 
Through the kindness of Dr. Walker- Arnott and Professor 
Busk, I have had lately placed in my hands some small frag- 
ments of a Laomedea 
from Vancouver's 
Island, with nume- 
rous specimens of a 
Triceratium attached 
to it, and the frus- 
tules are clearly united 
by their angles into a 
zigzag filament, as in 
the annexed figure, by 
a short thick gelati- 
nous cushion or stepis, 
exactly as occurs in 
Amphitetras. In 
some few cases I have 
found frustules subdi- 
viding, and the per- 
sistence of the connecting membrane as in Biddulphia, is 
another most important fact to be taken into consideration 
in locating the genus in any natural arrangement of the 
class; and shows, I consider, that there can be now little, 
if any, hesitation in removing it from Professor Smith's 
first sub-tribe, and placing with its natural allies in his fourth. 
With respect to the species to which these frustules can be 
referred, there appears to be some little confusion, which it 
would be well to clear up. Mr. Brightwell, in a paper dated 
June, 1853, has described a species under the name of Tri- 
ceratium Arcticum-\, but appears to have made some error in 
the measurement, as, though stated to be only ^joth of an 
inch in diameter, the valves which he kindly sent to me have 
distinct, though small, reticulations, and the diameter is^^^th 
* ' Mic. Jour.,' vol. vii, Trans. M. Soc, p. 21. 
f ' Mic. Jour,,' vol. i. p. 250, and t. iv, fig. 11 a and b. 
VOL. VIII. h 
