Structural Variation among the Difflugian Rhizopods. 238 
same time that one of the angles is truncated obliquely, thirdly, 
plates approaching in outline rectangular prisms ; and lastly, 
bodies in which the tendency to crystalline outline is lost and 
they pass into oblong colloid discs somewhat depressed at their 
centre. 
We now come to the forms shown at figs. 28 & 29, in which 
the surface of the tests is studded with the oblong bodies, some 
of which, however (as is more manifest in the larger specimen), 
exhibit a faint but nevertheless definite approach to elongated 
hexagonal prisms*. These measure about ~,!,;th of an inch in 
length. They are unmixed with any other bodies, and arranged 
side by side with a certain degree of regularity. In these the 
central depression is also distinct. The larger specimens (fig. 29) 
are from Greenland. The smaller, in which the dises are not 
more than about ;!;,th of an inch in length, are from Hamp- 
stead. 
In fig. 33, which also represents a tolerably frequent variety 
of test, the discs attain their maximum of regularity as regards 
shape: all are circular, or very nearly so, and exhibit the cen- 
tral concavity in a very marked manner; indeed they resemble 
blood dises in several particulars, but of course the resemblance 
is merely apparent. They are of varying sizes, the largest 
averaging about ~1,,th, whilst the smallest are not more than 
s=eiscth of an inch in diameter, the larger ones being generally 
surrounded by groups of the smaller, although, as seen in a 
number of specimens, there is evidently no particular order in 
which they are arranged, beyond that resulting from their taking 
up positions side by side, and all, without exception, resting on 
their flat surfaces. 
But it is in the series of forms already referred to as built up 
in a great measure of minute diatoms that we find the clue to the 
origin of the colloid dises and rectangular plates. In figure 32 
the diatoms, as will be seen, are interspersed amongst the circular 
dises. The very important fact reveals itself, however, that 
some of the diatom-valves are becoming gradually metamor- 
phosed, that is to say, exhibiting a gradual passage from the 
typical outline of the little Hunotia to one more closely ap- 
proaching an irregular cylinder, the cylinder then passing into 
the elongated disc, which is at times distinctly hexagonal, and 
finally the elongated disc passing into the circular one. The 
specimen figured is not so calculated to show the transitionary as 
the unmetamorphosed state of the diatoms; but in some indivi- 
duals, and more espécially in crushed specimens of these tests, 
every stage of transition may be clearly distinguished. As 
* It is not improbable that the tendency to assume this hexagonal form 
may result from pressure of the discs one upon the other. 
Ann. & Mag. N. Hist. Ser.3. Vol. xin. 16 
