400 
Transactions of the Society. 
X. — Development of the Young Valve of Trachyneis aspera Cleve. 
By Thomas Comber, F.R.M.S., F.L.S. 
{Read 15 th May , 1895 .) 
Plate VIII. 
The structure of the valves of diatoms has from time to time engaged 
the attention of the observers of those organisms, and has, indeed, 
occasionally been the subject of active controversy ; but, so far as I 
am aware, no attempt has hitherto been made to study the develop- 
ment of the valves, to investigate the various stages through which 
they pass in the course of their silicification, or even to collect facts 
which will serve to elucidate the process of formation. 
It will, therefore, perhaps be of interest to the Society to have 
placed on record, for the benefit of future students, some observations, 
which I have had the opportunity of making, on the development 
of the valve of Trachyneis aspera Cleve, perhaps better known under 
its former generic name as Navicula aspera Ehrenberg, or to British 
diatomists as Stauroneis pulchella Smith. Although they refer to 
only a few closely allied forms, they serve to illustrate the successive 
stages through which a diatom- valve may pass in the course of its 
formation ; and are possibly the more instructive, because in this 
group, which Cleve has recently separated under the generic name 
of Trachyneis , the valve possesses a highly complex structure. Cleve 
describes it as consisting of “ an interior coarsely dotted striation, a 
median stratum of more or less transverse flexuose strong costae 
anastomosing where they bend towards each other, and thus forming 
a network of diamond-shaped or rectangular alveoli, and an exterior 
stratum with very fine puncta forming longitudinal, sometimes 
slightly oblique, fine striae.” 
It is well known that by far the most usual mode of multiplication 
in diatoms is by subdivision. The two valves of a frustule separate 
from each other, their connecting zone gradually widening to admit 
of their doing so. When there is sufficient space between them, two 
new valves are formed inside, each facing one of the old valves ; and 
when the new valves are sufficiently developed, the parent frustule 
EXPLANATION OF PLATE VIII. 
Fig. 1. — Frustule of Trachyneis aspera var. ohlonga Cleve, undergoing subdivision. 
X 250. 
„ 2. — Fragment of one of the inner valves of the above frustule. X 1000. 
„ 3. — Immature valve of T. aspera var. pulchella Cleve. X 1000. 
„ 4. — Eye-spot layer of the mature valve of the same, x 1000. 
„ 5. — Mature valve of T. aspera var. ohlonga . x 1000. 
,, 6. — Mature valve of a form of T. aspera. x 1C00. 
„ 7. — Mature valve of T. aspera var. Schmidtiana Cleve. x 1000. 
