384 MR. G. 8. WEST ON 
viduals. The smallest specimen is from Minnesota, U.S.A.: 
long. 102 y, lat. 98 u; the largest from Capel Curig, N. Wales: 
long. 138 u, lat. 129 p. 
6. MICRASTERIAS DENTICULATA, Breb., in Mém. Soc. Acad. 
Sc. art. Falaise, 1835, p. 54, t. 8.—This species, although not 
quite so generally abundant as M. truncata, is by no means 
uncommon, and has a world-wide distribution. Many variations 
have been described in the lobulation of the margin of the cell 
and in the form of the polar lobes ; but I will confine my remarks 
to some other peculiarities frequently exhibited. In 1862, 
Archer described a species of this genus under the name of 
M. Thomasiana (Proc. Dubl. Nat. Hist. Soc. p. 72, t. 2. ff. 1-5; 
Micr. Journ. p. 259, t. 12), and, although a typical specimen of 
Archer's species possesses quite sufficient specifie differences to 
separate it easily from M. denticulata, yet many intermediate 
forms are constantly met with. This fact induced Jacobsen 
(Botan. Tidsskrift, vol. viii. 1875, p. 187) to regard M. Thomasiane 
as forma Thomasiana of M. denticulata. Whether he was fully 
justified in so doing may long remain an open question, but 
there is no doubt that the main distinguishing feature of M. 
Thomasiana (the three large protuberances at the base of each 
semicell) is subject to very considerable variation. Figs. 2-5, 
Pl. 9, represent a few vertical views of intermediate forms, 
which show the extent to which the median protuberances may 
he developed or suppressed. ‘he extremities of these pro- 
tuberances may be rounded, papillate, or even bidentate (cfr. 
figs. 3, 5-8), and it not unfrequently happens that more than 
three are present on each side (fig. 5). The latter is, moreover, 
a form connecting M. denticulata, Bréb., with a species recently 
described by Bisset in Scott. Nat. 1893, p. 174, t. i. f. 2, as 
M. verrucosa, Individuals are often met with in which only two 
of these basal protuberances are present (figs. 6 & 7); and the 
smaller denticulations on the surface of the cell, which have 
a definite arrangement in the typical form of M. Thomasiana, 
are frequently quite irregular in their disposition (figs. 1, 7, & 8). 
The lobulation of the margin of the cell is described as being 
more acutely dentate in M. Thomasiana than in M. denticulata ; 
but specimens of the latter species often possess an acutely 
dentate lobulation, without any of those more salient characters 
appertaining to M. Thomasiana. 
