Carter . — Studies on the Chloroplasts of Desmids. /. 245 
was all the more striking because an unusual specimen of Eu. ansatum 
(Group I) was also encountered which was almost identical with the 
abnormal specimen of Ett. eras sum described above. 
The Chloroplasts of Eu . verrucosum. 
This species differs from all the other species examined in having two 
chloroplasts in each semi-cell. In each chloroplast there is typically one 
large pyrenoid, and the general appearance of the cell is very suggestive of 
certain large species of Cosmarium. Each chloroplast is provided with an 
axis, which, beginning at the isthmus, stretches diagonally towards the 
incision between the apical and lateral lobes; cf. Fig. 91 . This axis con- 
tains a single large pyrenoid, and around it are arranged about four or five 
irregular ridges which radiate towards the front and side walls of the cell 
(Figs. 90 and 92 ). The peripheral edges of the ridges are usually very 
little ornamented. 
Thus the chloroplast of Eu. verrucosum is very unlike that of any other 
species examined, and it is therefore very difficult to explain its relationships. 
It is possible, however, that the examination of other more closely allied 
species would help to make clear its affinities. 
VII. The Chloroplasts of the Genus Xanthidium. 
In a few species of the genus, the form of the chloroplast can be made 
out from living material, and the chloroplasts of one or two species were 
figured by Delponte (1873). The structure of the chloroplast and the 
position of the pyrenoids were also indicated in several species by W. and 
G. S. West (1904-11). 
Boldt (1888) attempted to subdivide the genus according to the dis- 
position of the chloroplasts, and instituted the sub-genera Euxanthidium 
and Centrenterium to include those species having parietal and axile 
chloroplasts respectively. 
Larsen (1907) described and figured X ’. groenlandicwn , Boldt, indi- 
cating that in its chloroplasts this species agrees fairly well with certain 
species of the genus having a distinctly parietal chloroplast, whereas Boldt 
placed it in Centrenterium. 
During this investigation the following species were examined :■ — 
X. aculeatum , Ehrenb., X. acanthophorum , Nordst., X. hastiferum , W. B. 
Turn., X. Brebissonii> Ralfs, X. antilopaeum , (Breb.) Kiitz., X. cristatum , 
Breb., X . subhastiferum, var. Murrayi , W. and G. S. West, X. fasciculatum, 
Ehrenb., and X. armatum , (Breb.) Rabenh. The chloroplasts of these 
species were found to be of two distinct types, either axile or parietal, but in 
at least one species the form of the chloroplast is variable, and transitions 
from the axile to the parietal condition were observed. 
