232 Dr. G. C. Wallich on the Extent and Causes of 
series of forms was detected by me last autumn in the Hamp- 
stead material, and described and figured in ‘The Annals? for 
December 1863, under the name of Difflugia pyriformis, var. 
symmetrica (loc. cit. p. 458, plate 8. fig. 16). For facility of 
reference, I have again figured this form in the plate illustrative 
of the present section of my subject (figs. 26 a, b,c, &d). As pre- 
viously stated, the test, instead of being built up of irregular mi- 
neral particles so as to impart a rugged outline, is entirely made 
up of hyaline rectangular plates, arranged with the greatest re- 
gularity in consecutive transverse and longitudinal rows—the 
smaller plates being thus disposed towards the extremities, 
whilst the larger ones occupy the central and widest portion of 
the structure. It was stated at the same time that the chemical 
composition of these plates had not been ascertained by me (the 
reason being that they do not occur in sufficient quantity to 
render experiment practicable), but that there was ground for be- 
lieving their nature to be crystalline and siliceous—firstly, be- 
cause the plates resisted the effect of the heat employed in 
mounting the specimens, and their angles were most perfect, 
and secondly, because they presented no coloration when seen 
with the aid of the polarizer. 
Since the date of my first examination of this form, I have 
succeeded im procuring a considerable number of similar speci- 
mens from the same locality ; but, beyond confirming my pre- 
vious statement im every particular, I have nothing to add, save 
that the test is more or less compressed laterally, whereby an 
elliptical outline is given to the section and aperture (figs. ¢, 
d, & 2), } 
The true nature of these rectangular plates will, I believe, 
become manifest as I proceed with the description of the transi- 
tional states which intervene between this, the most aberrant 
kind of composite surface-configuration to be met with amongst 
the tests of the Difflugia, and the least aberrant forms, which are 
represented at figs. 830 & 31. 
Fig. 27 represents the first form to be noticed. In this there 
is an admixture of at least four apparently distinct sets of bodies 
attached to the surface of the test. These are not arranged with — 
the symmetrical order observable in the rectangular plates ; but 
this may be accounted for, inasmuch as the specimen (which in 
this instance is a solitary one) was mounted in balsam in the 
material in which it occurred before I had become aware of its 
existence ; and hence, in all probability, the pressure of the glass 
cover of the slide caused a certain degree of displacement. Be 
that as it may, it is quite manifest that we find associated toge- 
ther in the same individual, first, rectangular plates, secondly, 
others in which the plates are produced in one direction at the 
