THE FRESHWATER PLANKTON OF THE SCOTTISH LOCHS. 501 
utrobique; semicellulze a vertice visze ellipticee, tuberculo mediano obtuse mamillato 
utrobique, spinis plerumque in serie unica. 
Long. sine spin. 46-50, cum spin. 61-71; lat. sine spin. 42-49 w, cum spin. 
69-82; lat. isthm. 12-14; crass. 31; long., spin. 13°5—-19 x. 
Hab.—Common in the plankton of many lochs in Inverness, Ross, Sutherland, and 
the Outer Hebrides. 
Instead of the normal pair of spines at each upper angle of a semicell, there is in 
this variety only a single spine, and the pair of spines which should be attached side by 
side at the lateral angles are here separated some distance apart, and placed more or less 
vertically over each other. The semicells are therefore not so angular as in typical 
X. antilopaum. The length of the spines varies in different specimens of the var. 
Hebridarum, but otherwise the distinguishing features are retained very constantly. 
Genus Arthrodesmus, Ehrenb., 1838. 
19. A. Incus (Bréb.), Hass. 
A number of very large forms of this species were observed from Loch Cuthaig, 
Lewis. The spines were in all cases widely divergent, and in some specimens the cell- 
wall possessed minute scrobiculations. Long. sine spin. 25-32°5m; lat. sine spin. 
21-26»; long. spin. 21-32; lat. isthm. 7°5-8°5 wu. 
Var. Ralfsu, W. & G.S. West. [A. Ralfsw, West. | 
Forma spinis brevissimis ; long. 15°44; lat. 15 4; lat. isthm. 7°5 »; long. spin. 
3-3°8 wu (Pl. 7, fig. 10). 
Hab.—Lochs Fadaghoda, Roinebhall, and Stranabhat, Lewis, Outer Hebrides. 
Var. longispinum, var.n. [A. Incus, Wolle, Desm. U.S., 1884, t. 24, f. 3.] 
Var. spinis perlongis, multe sursum divergentibus. 
Long. sine spin. 14; lat. sine spin. 11°5 w; lat. isthm. 5°7 «4; long. spin. 21-25 » 
eel, 7, fig. 22). 
Hab.—Small loch near Cearnabhall, Lewis, Outer Hebrides. 
Genus Stauraustrum, Meyen, 1829. 
20. St. inelegans, sp. n. (Pl. 7, figs. 11, 12). 
St. mediocre, paullo latius quam longum, profunde constrictum, sinu latissimo et 
obtuso, isthmo breviter cylindrico ; semicellule late subtriangulares, marginibus in- 
ferioribus subrectis vel leviter biundulatis, apice in medio convexo, angulis productis 
acutis vel apiculatis (rare subobtusis); a vertice vise triangulares, lateribus convexis 
in medio, angulis submamillatis et leviter apiculatis ; membrana glabra. 
Long. 53°5-60 u; lat. 51-62; lat. isthm. 9-10 u. 
Hab.—Loch Fadaghoda, Lewis, Outer Hebrides. 
This remarkable Stawrastrwm is somewhat variable in its characters. ‘The angles 
of the semicells are generally produced, and either acute or apiculate, but in some 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART III. (NO. 21). 74 
