82 
Dr. Wallich, on the Polycystina. 
externally, but compressed so as to become more or less 
perfectly discoidal, or, instead of being either truly spherical 
or discoidal shape, the form assumed is distinctly stellate. 
But even in the most aberrant of these sections the ompha- 
lostype remains unchanged in outline, and the transition is 
effected by gradations which only become obscure in those 
last-formed portions of the structure which are furthest re- 
moved, radially, from its central axis. 
In the Monodinal subdivision, on the other hand, there is 
never such an apparent variation ; for although monstrosities 
are to be met with now and then, in which development of 
siliceous material has taken place laterally, it is impossible to 
mistake these as having been formed in the normal course of 
growth of the organism. Moreover, they never assume the 
characters of regular chambers. 
All subsequent deposits of silex, whether in the shape of 
foraminated chambers or spines, or portions of structure 
not referable to one or other of these kinds, take place on 
the same plan, namely, by deposit of silex at right angles to 
the axis of growth of the part immediately in question. In 
the formation of the chambers the deposit usually goes on 
from a number of points simultaneously around the free 
margin, the points becoming filaments, and the adjacent 
filaments ultimately anastomosing, or rather coalescing, as 
soon as they come in contact. As already stated, the spines 
are never tubular, the appearance of tubularity in the spines 
of some genera (as, for example, certain adult specimens of 
Podocyrtis) being due to the existence of short longitudinal 
furrows and buttresses on their inner aspect, where generally 
may be seen an aperture around a portion of the margin of 
which the base of the spine has taken its rise. When loops or 
festoons occur the process is still the same, as these may be 
seen in every stage of growth from the first projection of a 
minute filament to the stage at which the coalescence would 
have become complete had the protecting and formative 
living sarcode been left to fulfil its office. In short, the process 
may be familiarly likened to that by which the glass- worker 
extends his plastic and half molten material from point to 
point when manufacturing a miniature basket-work. Of 
course the thickening of each portion is by subsequent 
(Jeposit around the original thread. 
There occurs, in some of the oceanic deposits containing 
effete skeletons of the Polycystina, an abnormal growth in the 
discoidal group, which is certainly unique and has heretofore 
been overlooked. I allude to the presence of a short canal, 
which is found projecting from one portion of the circumfer- 
