I. ROBINSON—THE DIATOMACE2E. 
3 
opposite directions, and somewhat resembling the letter S. Others 
are irregular in form, and one of the most beautiful found in our 
own locality, Campylodiscus costatus, bears a very close resemblance 
in shape to a saddle. 
But an equal, if not indeed a greater interest attaches to the 
markings found upon their surfaces. These are of great diversity, 
though constant in the same species. In some they consist of very 
fine lines called striae, perfectly straight and parallel, and executed 
with the greatest mathematical precision. These lines when sub¬ 
jected to examination under a sufficiently high power are found in 
some cases to consist of separate bead-like dots, and are in conse¬ 
quence termed moniliform, while others appear simply as ridges on 
the surface. If we examine a valve of Pleurosigma attenuatum, a 
very frequent form in this locality, we shall find that it affords an 
excellent illustration of the dotted lines just referred to. The dots 
form rows corresponding lengthwise with the curve of the valve, 
and they are so placed that shorter rows are also formed at right 
angles across its surface. It is found that there are 40 of these 
dots in one-thousandth of an inch. If however we examine a 
marine species of the same genus, Pleurosigma angulatum, we shall 
find that the rows are arranged diagonally, and cross each other 
obliquely at an angle of about 60°. The dots also are very much 
closer together and count 52 to the thousandth of an inch, and, 
although a single frustule will show many thousands of them, yet 
so perfect is the arrangement that not one can be detected as mis¬ 
placed. When examined under a very high power these separate 
dots are found to be of a hexagonal form. These objects are fre¬ 
quently made use of as tests of the quality of microscopical lenses. 
The most crucial test, however, in this way is the valve of Amphi- 
pleura pellucida, which also is frequently found near Hertford. In 
this species the lines are of extreme delicacy and so closely placed 
that about 100 are found to exist in one-thousandth of an inch, and 
even these lines are found to consist of dots placed at regular 
intervals, at the rate of 84 to one-thousandth of an inch. 
The cellular arrangement of many diatoms also claims our notice. 
Some of these minute plants seem to be endowed with the ability to 
solve most difficult geometrical problems. Tor instance a triangular 
species, Triceratium Favus, fills up its central part with a most 
perfect series of hexagonal cells, placing however a pentagon here 
and there in the solution of the difficult question of adapting the 
two geometrical figures to each other. Many of the circular forms 
are also filled up in the same manner. 
The minute size of these organisms is very remarkable, 
especially when considered in connexion with their marvellous 
perfection of form and detail. The actual size of some of the 
individual forms of the largest species is about one-fortieth 
part of an inch in length, but these must be looked upon as 
veritable giants amongst their fellows. The majority are very 
much smaller. Some of the circular and oval forms common 
in this district measure only about one-thousandth of an inch 
