134 
Wallich, on the Diatom-valve. 
taken place^ as just stated, equalling in dimensions the largest 
specimens of the species to which they belong. And, lastly, 
we never meet with such differences in size and markings as 
would, of necessity, result, did growth continue in the 
terminal parent valves of an elongated filamentous species ; 
whilst the central or most recently produced valves exhibit 
only the size and markings attained by the parent valve at 
the period at which the first occurrence of division intervened. 
A further and most remarkable confirmation of the view, 
that growth does not take place in the valve after its libera- 
tion from the parent connecting zone, is, I submit, derived 
from an abnormal form of Triceratium favus, a figure and 
description of which are given by Mr. Brightwell, in the 
^ Quarterly Journal of Microscopical Science,^ vol. i, p. 246. In 
this specimen, an oblong portion, equal in extent to about one 
third of the entire valvular surface, is cleft out, as it were, 
from one of the angles. It is evident that, from whatsoever 
cause this configuration occurred, that cause must have taken 
effect whilst the valve was still in the plastic condition, to 
which reference has been made; for the cleft margin is 
fringed by a regular series of quadrangular cellules, such as 
we frequently observe along the inflected edge of the valve in 
the species under notice. As the specimen must have been 
subject to the action of acid or heat, before its intimate struc- 
ture could have been examined and figured, it is equally 
evident that the valve had attained its mature and perfected 
condition. It should be borne in mind that the valve is of 
normal outline and configuration on the remaining surface. 
There is no projection from the sides or angles, indicating 
that the object to which the abnormal development was due 
had taken effect after the complete valve had been deposited ; 
but, on the contrary, it is clear that such object must have 
presented an obstacle to the complete development of the 
valve whilst it was retained within its parent connecting 
zone. From the shape of the emarginate portion, it would 
appear to have been produced by growth taking place around 
some substance, such as a calcareous or siliceous spicule. 
From its not having been retained in situ, we may infer — 
either that, being siliceous, it had broken its way out at the 
deficient angle ; or, being calcareous, that it had been dissolved 
during the operation of cleaning by acid. It is hardly possible 
to conceive that an object could pierce the already perfected 
and consolidated diatom-valve. But, supposing that possible, 
it is certain that fracture must have resulted, or that an 
extent of valvular surface must have been displaced of equal 
bulk to the emarginate space. 
It may be asked, then, to what cause are we to attribute the 
