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2. CoUetonema vidgare, Thw. Frond filiform, occasionally divided,
gradually acuminate, containing one or two rows of frustules in
regular file ; V. elliptical-lanceolate, slightly contracted towards
the obtuse extremities; striæ 72 in -001". Length of frustule
•0018" to -0030". Breadth of V. -0004" to -00055". v.v.
Thw. Ann. ser. 2. vol. 1. pl. xii. H.
Freshwater. Near Bristol, Dec. 1847, Mr. Thwaites. Near Lancaster,
April 1852, Mr. Johnson. (Falaise, M. De Brebisson. Cascade de la
Venüère, Mont Dore, June 1854, W. Sm.)
This is by no means the common species represented by Mr. Thwaites, who
says that “ it occurs during the spring in almost every ditch and running
stream,” a mistake which appears to have arisen from confounding it with
the next species, which is far more general. To distinguish between the
two requires the careful use of the microscope, as the form of each is so
nearly alike. The principal difference is to be found in the character of
the striæ, which are tolerably conspicuous in C. neglectum, and extremely
faint and delicate in C. vulgare. Mr. Thwaites describes three varieties of
the present species, viz. a. rimilosum, ß. lacustre, y. effusum-, hut the distinctions
between them depend mainly upon the stage and place of growth ;
the first and last occur in streams, var. a. with a divided frond, and
var. y. forming a gelatinous covering to stones, with no appearance of
separate filaments,—while var. ß. is characterized by a simple filament, and
IS met with in still water, or where there is only a slight current ; the frustules
of all three are described as havmg “ a lanceolate form, suddenly
narrowed towards the apices.” Not having found the species in more than
one locality, I am unable to confirm this account.
Plate LVI. 351.
3. CoUetonema neglectum, Thw. Filament slightly divided, obtuse,
containing numerous and closely packed frustules ; V. elliptical
lanceolate ; extremities obtuse ; striæ 32 in-OOl". Length of
fru stu le -0015" to -0024". Breadth of v alve-00035" t o -00045". v.v.
Thw. Ann. ser. 2. vol. 1. pl. xü. J.
Fresh water. Near Bristol, Dec. 1847, Mr. Thwaites. Near Lewes, April
l8ol, Nov., Dec. and Jan. 1853, TF.Sm.
Plate LVI. 352.
4. CoUetonema subcohærens, Thw. Frond globose, gelatinous,
pervaded by files containing from one to five rows of frustules ;
V. elliptical-lanceolate, with rounded extremities ; striæ 28 in "OOl".
Diameter of frond from -2" to -6".
•0015". Breadth of V . -00035". v
Length of frustule -0008" to
Thw. Ann. ser. 2. vol. 1. pl. xii. G.
Freshwater. North Eiver near Wareham, Dorset, July 1847 and May 1863,
W.Sm.
This species bears a slight resemblance in the character of its frond to
Berkeleya fragilis, but differs in the form of its frustule and its freshwater
habitat. The globose frond originates in the same way as in Berkeleya, by
the coherence of the mucus-tubes or filaments ; but, unlike that species, the
mucus continues in the present to increase in volume, and always extends
beyond the extremities of the rows of frustules ; while in Berkeleya the filaments
are free at their terminations, being continued beyond the mass of mucus.
Conjugation has been observed in the present species, and takes place within
the frond precisely in the same manner as in the free forms described under
the first tribe.
Plate LVI. 353.
G e n u s 56. SCH IZO N EM A ,
Frond gelatinous or submembranous, filiform, or by coherence leaflike,
usually much divided ; frustules naviculoid, arranged in one
or more files in linear series within the substance of the frond;
valves elliptical, or lanceolate, striated. (All the species marine.)
The fronds in this extensive genus were among the earliest Diatomaceous
organisms recognized by naturalists, and have been the perplexity
of all subsequent observers. Their external form, size and
colour vary with age, season and locality, and in consequence any characters
based alone upon these particulars are uncertain and deceptive.
Now these were the characters most relied upon by the earlier writers
in their specific arrangements, and hence the descriptions of one
author were often irreconcileable with those of another, aud the
species became inextricably confounded. Later observers endeavoured
to clear up this confusion by a closer examination of the
structure of the frond, and it was thought by some that the species
might be arranged under tw’O genera, as they presented one or more
series of frustules included within a single gelatinous or membranous
tube, or contained in secondary tubes, enclosed within a general