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24 Rhodora ` [FEBRUARY 
may be simple or forked, and usually have the edges crisped; when 
they are simple, there is quite a resemblance to a clump of individuals 
of Enteromorpha Linza (L.) J. Ag.; when forked, the resemblance is 
equally marked to Ulva fasciata Delile; the laciniae are sometimes 
quite palmately arranged. The frond of M. Lactuca is somewhat 
thicker; 20-25 p, as against 15-20 и in M. Grevillei; the structure of 
the former is more distinctly parenchymatous, and the texture less 
soft and lubricous. In a cross section of a vegetative frond the cells 
show much alike, horizontally elongate, occupying about two thirds 
of the thickness of the frond. 
In Wittrock's monograph, Plate IV, fig. 14, c, represents a fruiting 
frond, with the characters of M. Lactuca; in M. Grevillei, as observed 
by the writer, the fertile portion of the frond puts on quite a different 
appearance; the membrane becomes thicker and more gelatinous, the 
cells elongate vertically to the surface of the frond, finally assuming the 
palisade form characteristic of M. fuscum, though on a smaller scale; 
as the spores are discharged, the membrane melts away, and there is 
nothing of the persistent empty tissue, shown in M. Lactuca, which 
was the principal character for the exclusion of the species from 
Monostroma by Thuret and Le Jolis. The writer does not claim 
the original discovery of this form of spore production; it has been 
noted by Rosenvinge ! but he thinks that this is the first suggestion 
that it may be a distinguishing character between the two species. 
Specimens in this fruiting condition have been distributed as P. B.- 
A., No. 1467. 
The question of how far related forms, evidently closely connected, 
are to be distinguished as species, will probably always be a matter of 
discussion; so much depends on the way of looking at the matter. 
Jónsson,” refers to the writer’s arrangement of M. Grevillei and allied 
forms,’ as follows:— “I cannot admit Collins to be right in dividing 
M. Grevillei K. Rosenv. into two species: M. Grevillei Collins includ- 
ing var. Vahlii К. Rosenv., and M. arcticum Collins including var. 
intestiniformis К. Roseny. ‘The limit between the two species as 
understood by Collins, is as indistinct as the limit between the main 
form of the species and the included varieties. If closely related forms, 
which run into each other, are not to be regarded as belonging to one 
! Gronlands Havalger, Meddelelser om Gronland, Vol, III, p. 948. 
2 The marine algae of East Greenland, Meddelelser om Grønland, Vol. XXX, p. 65, 
з The Ulvaceae of North America, Ruopora, Vol. V, p. 13. 
