64 Transactions of the Royal Microscopical Society. 
opinions at the present time entertained by the principal writers 
on the Diatomacex, upon the questions about to be discussed. 
This done, it shall be my endeayour—l. To denote the extent to 
which the conditions invariably attending the development of every 
new or “secondary” valve of a frustule that has once undergone 
the process of division may operate in modifying its proportions 
and its markings. 2. To supply some missing links in the history 
and actual functions of the “ Sporangial Frustule.” And lastly, to 
inquire briefly how far any modifications, engendered by any or all 
of these combined causes, can be regarded as detracting from the 
value of the diatom valve as a definite and trustworthy “test” of 
the defining power of microscopic objectives. 
In the 1875 edition of the ‘ Micrographie Dictionary’ (p. 238), it 
is stated that the frustule of the Diatomaceex consists of two usually 
symmetrical portions or valves, which are in contact at their 
margins with an intermediate piece, the hoop, variable in breadth 
according to age, &c., and, when very narrow, forming a mere 
junction line, which is called the line of suture. In some, this hoop 
is a simple filament; in others, it is broad and marked like the sur- 
faces of the valves; whilst in others, again, the hoops are arranged 
like the leaves of a book, each with a round or oval aperture 
in the middle, so that the cavity of the frustules is divided into 
loculi, and the frustules themselves are to be regarded as compound. 
During division the hoop undergoes an increase of width, and thus 
removes the two valves to some distance apart.* Sometimes the 
hoop consists of two pieces, one overlapping the other. The valves 
of Melosira and Isthmia are described as gradually separating 
from each other during division, and remaining connected by the 
simultaneous widening of the hoop. The history and ultimate 
fate of the hoop seem to be variable. Sometimes it becomes 
solidly silicified, but not much expanded in breadth, and falls off 
when the two frustules are complete, allowing them to separate; 
this being the case in Gyrosigma (sic), and probably in all the 
allied forms. Perhaps the most remarkable development of the 
silicified hoop occurs in Biddulphia, Isthmia, and similar forms ; 
the new half-frustules, formed inside the hoop, slipping out from 
at like the inner tubes from the outer case of a telescope. In 
Melosirva, the hoops appear to keep the new frustules united for 
some time. 
* It will be hereafter shown that the process is strictly the converse of that 
which is here described, namely, that the siliceous valves are foreed asunder by 
the increase of their soft contents during the progress of division, each of the 
connecting zones being of course drawn back by the valve to which it is adherent ; 
but, when the separation of the two parent valves becomes extreme, the connecting 
zones each increasing by the secretion of fresh siliceous matter at their free over- 
lapping edges, as before described. Of course, therefore, it is an error to say that 
the valves of Melosira and Isthmia, or any other genus, remain connected through 
the simultaneous widening of “‘ the hoop.” —G. C. W. 
