conclude, as I have stated in the Introduction, that the lines arise
from internal structure ; that the appearance hoth of perforations and
elevations may be produced in the same object by a slight alteration
in the focus of the instrument ; and that such appearances are merely
optical illusions, produced by the reflection and refraction of the
rays passing through the minute cellular structure of the siliceous
epiderm.
I t is evident that no optical effect can confer upon circular elevations
or perforations hexagonal outlines, while it is easy to see how
the latter, when out of focus, or viewed by an object-glass of imperfect
definition, may assume the semblance of rounded beads or dots.
And, moreover, as the view which regards the striæ of Pleurosigma as
arising from the cellular arrangement of the epiderm, harmonises the
structure of this extensive genus with that of the other members of
the family, it seems recommended to our acceptance by claims too
strong to be resisted.
Nothing is known of the Reproduction, properly so called, either
of this or of the three preceding genera, multiplication by self-division
being the only mode of increase hitherto detected. While this process
is actively going forward, the mucus generated by the dividing
frustules is often so considerable as to produce the appearance and
effect of a distinct frond, which assumes the form of a thin pellicle of
some little tenacity. At other times, when the mucous secretion does
not assume the continuity of a pellicle, it invests the individual frustule
with a transparent envelope, which has the appearance of an exterior
membrane, and has been sometimes mistaken for such. On
one occasion I also met with the frustules of P . Hippocampus enclosed
in mucous or gelatinous tubes, precisely like those of a Colletonema ;
but these conditions must be regarded, for the present at least, as
temporary or accidental, and cannot be admitted into the specific or
generic descriptions.
Owing to the compression of the valves, the scanty development
of the connecting-membrane, and the elongate form of the frustule in
the present genus, the front view is either of a linear or linear-lanceolate
form ; two examples are given in PI. X X III. 203 & 207 ; but
I have omitted a special notice in the case of each species, as I have
not found this feature of much assistance in their discrimination. On
the other hand, as the colour of the dry valve is often conspicuous,
and seems to depend on the structural character of the silicified membrane,
and also supplies a ready means of discrimination, I have endeavoured
to record it with as much precision as possible. Prom
notices forwarded by Mr. Beck, I am disposed to conclude that a
yellow tint m the valve indicates the presence of a distinct hexagonal
structure in its cellular condition, and a purple colour an absence of
such character.
■ In several species the striæ are so closely arranged—or, in other
words, the cellular structure is so minute,—that the power I have ordi-
nanly employed has proved insufficient for their resolution. In such
cases I have availed myself of Mr. Beck’s aid, who has succeeded
with an object-glass of ^-inch focal length and angular aperture of
120 , m resolving the lines on most of the species alluded to.
S ect. I. Striæ oblique.
1. Pleurosigma formosum, W. Sm. V. hnear-lanceolate, twisted
flexure considerable, extremities obtuse; median line not central;
colour light chestnut-brown ; striæ 36 in -001". Length -0141" to
•0178". v.v.
W. Sm. Ann. 2nd Ser. vol. ix. p. 5. pi. i. 1. Prit. Anim. xix. 1.
C^7pNng.
Plate XX. 195. Frustule with endochrome. Frontispiece, fig. CXCV.
2. Pleurosigma decorum, n. sp. V. lanceolate, acute, angular,
twisted, flexure considerable; median line not central; colour Dale
chestnut; striæ 36 in -001". Length-0098" t o -0125". v.s.
a w , . , A.g. .85,.
Plate XXI. 196.
3. Pleurosigma speciosum, IV. Sm. V. linear-lanceolate, obtuse;
flexure slight, twisted; median line subcentral; colour a pale
straw ; stnæ 44 m -001". Length-0083" t o -0152". v.s.
W. Sm. Ann. 2nd Ser. vol. ix. p. 6. pi. i. 3. Prit. Anim. xLx. 3.
Marine. Walton, Essex, Aug. 1851, 3Ir. C. Topping.
Plate XX. 197.