286 MR. G. S. WEST ОХ SOME CRITICAL GREEN ALGAE, 
An Alga has recently been described by Sehmidle', under the name 
of Didymogenes palatine, which bears much resemblance to Ajrchneriella 
subsolitaria. Из cells, which are mostly in pairs, are not bent anything 
like so much, and each possesses a chloroplast with one pyrenoid. T do 
not see how Schmidle finds any generic characters for Didymogenes palatina: 
but whatever it may be, it is not the Alga here described as Arehneriella 
subsolitaria. 
5. l'erRAEDRON. PLATYISTHMUM, nob. (= Cosmarium platyisthmum, Archer, 
1880). T. mediocre, subreetangulare, compressum, circiter tam longum 
quam latum, lateribus late excavatis cum isthmo lato et longo in centro 
cellule, apicibus convexis sed in medio leviter retusis, angulis lateralibus 
subproduetis et inerassulatis, cum fossa inconspicua longitudinaliter disposita 
in centro cellule репе tenus apice. unoquoque : membrana conferte punc- 
tulata, cum serie singula punctorum majorum trans partes latiores duas 
cellule. Cellula а vertice visa elliptica, polis attenuatis, excavatione minuta 
ad medium utrobique ; a latere visa elliptica. Chromatophora eum pyrenoide 
centrali. Long. 24-35 и: lat. 28-355 р: crass. 12:5-14 ри. (PI 21. 
figs. 36-39.) 
Hab, Near Rhiconich, Sutherland ( West). Ireland (2beeher). 
Some collections of Algze from the bogey hollows in the Lewisian Gneiss 
of West Sutherland, which were rich in Desmids of the Western British 
type, such us Docidium undulatum, Bail., Ретит adelochondrum, Elty., 
Huastrum intermedium, Cleve, №. pingue, Elfy., and Staurastrum elongatum, 
Barker, also contained a most peculiar species of the genus Tetraédron, И 
agrees with no described species of this genus, and I have not observed 
it from elsewhere. 
The cell is more or less quadrate when seen from the front, with а broad 
excavation at each side, which gives it the appearance of a specles of 
Cosmarium with a very wide and elongate isthmus. The apices are faintly 
retuse and the lateral angles are produced and somewhat attenuated, having 
a slight thickening at the extremity. Seen from the side the cell is 
elliptical ; from the apex it is elliptical with attenuated poles, and exhibits a 
slight indentation in the middle of each side. The cell-wall is finely 
punetate, and extending from angle to angle across each half of the cell 
is a row of rather stronger and more distant punctæ disposed in the form of 
ап are. The greater part of the living cell is occupied by a single large 
chloroplast, with several irregular ridges and a central pyrenoid (fig. 36). 
Several empty individuals were noticed which possessed a prominent slit 
down one face of the cell-wall, most probably indicating the place of escape 
of autospores (fig. 39). 
| W, Schmidle, “ Zur Kenntnis der Planktonalgen," in Hedwigia, xlv. (1905). 
