134 THE MICROSCOPE. 
while more or less elastic, are unmistakeable if present, is eminently 
well chosen and ought to stand. That the discoid character is as 
well marked a feature in this case as in that of Coscinodiscus or 
Actinoptychus, no one, I think, will doubt from an examination of 
the valves. The only other points of structure to be considered are the 
naviculoid marking, the raphe and the so-called nodules.. Although 
the naviculoid marking is remarkably uniform in all the valves, 
some thirty, which I have examined, I can find nothing to indicate 
that it is any more structural than the blank space in Stauroneis or 
Pinnularia, I believe that nothing in the growth or development of 
the valve depends on this naviculoid marking, and especially not on 
its shape; hence, that we may expect to find valves of Raphidodiscus 
with this nayiculoid space modified in form, as oval, circular, etc., or 
altogether wanting. The raphe, having a physiological function as 
a channel of communication between the interior and exterior of the 
frustule, is, of course, a structural feature. The nodules, although 
a recognized feature of classification, are commonly regarded as not 
essentially structural; indeed, it is only their extremely uniform 
persistence in the genera of the tribe Raphidiez that has caused 
them to be recognized as generic features. I believe, however, that 
the terminal nodules have, to some extent, structural relations, and 
that the same is true of the feet, horns, processes and spines of the 
diatoms generally, especially where they are well marked and 
uniformly present. I am aware that in many cases the spines are 
variable, and sometimes nearly obsolete; also, that in some cases the 
only difference to be discovered between two diatoms is the presence 
in one case of spines, often minute or obscure, and their absence in 
the other. I should in such cases regard the form with spines as 
the typical form, and the other an aberrant form in which a single 
feature has been suppressed. 
In Raphidodiscus the terminal nodules are, in my opinion, 
clearly spinous, the spine, although minute, being fully as well 
marked as in Actinocyclus, in which genus they form a generic 
character. In Biddulphia, Triceratium, etc., the spines, as well as 
the ‘‘ horns,” are clearly structural; in Stephanopyxis, Systephania, 
etc., they can scarcely be regarded otherwise; and in Stephanodiscus 
I am satisfied by observation that the spines fulfil an important part 
in the growth of the frustule, serving by the growth of their opposed 
extremities to push apart the two newly-formed frustules, and thus 
widen the connecting zone between the young valves. In Aulaco- 
discus the prominence of the spines and processes strongly supports 
the idea of their structural character; and I am satisfied that in all 
