RopER^ on Triceratium Arcticum. 
55 
cles of the ocean may be accumulated together into masses 
and rendered sufficiently coherent to admit of their reaching 
the sea-bottom without absolute disruption. 
Xanthidium. 
X. ? — Cell sub-spherical^ with from eight to ten long 
tapering spines_, the extremities of which are dichotomously 
divided. Endochrome (as taken from the Salpse stomachs) 
of a pale yellowish- green colour^ generally shrunk to a 
slight extent. Cell- wall exhibiting the primordial utricle and 
a stout horny external (cellulose) layer continuous with the 
spines. Diameter of cell '0012 ; total diameter '0038. 
(Fig. 23.) 
Bay of Bengal and Indian Ocean. April^ 1857. From 
Salpse. 
Fig. 24 represents X. vestitunij as copied from Mr. Whitens 
figure. 
Fig. 2 represents a frequent form of C. Sol, in which the 
proportion between the size of the disc and membranous ring 
approaches very closely to that seen in the fossil. 
On Triceratium Arcticum. By F. C. S. Boper^ F.L.S., &c. 
The genus Triceratium was first proposed by Professor 
Ehrenberg^ in a paper read before the Berlin Academy in 
1839^ to include a class of forms which he stated to be free or 
non- concatenate. 
Well known as the large and peculiar species of this genus 
have been,, from that time to the present, to those who have 
studied this class of algse, it is a remarkable fact, that all the 
well-known species, such as Triceratium favus, alternans, 
striolatum, &c., which occur in the tidal harbours and estuaries 
of our coasts, have been met with merely as scattered speci- 
mens in the mud, and never in a living state, or attached to 
larger algae, as is frequently the case in the genera Biddulphia, 
Amphititres, and Isthmia. 
Belying on this negative evidence, all writers on Diato- 
macese have followed the German Professor in describing the 
genus as non- concatenate, and in the synopsis of Professor 
