ON THE RADIOLARIA AS AN ORDER OF THE PROTOZOA. 381 
bers, and the spines. 13 a. The omphalic chamber just developed, 
and the projecting spines. 13 b. The same advancing towards 
the completion of a second chamber. 
Fig. 14. Sectional view of one of the discoidal Haliommata. The spherical 
omphalic chamber shown at its centre, with spines projected 
from it only in one axial plane ; the peripheral spines taking 
their rise altogether from the external surface of the shell. 
,, 15. Side view of Haliomma amphicliscus (Muller), one of the Dicho - 
dinal series, showing the interrupted growth of the external 
chamber which is always more or less compressed. An internal 
chamber is seen within the outer one, and two spines extending 
from its long axis. Of course in this the omphalostype could not 
be represented. But if seen, it would invariably be of the per- 
fectly spherical form characteristic of the Cyclodinal division. 
„ 16. A Haliomma — showing the second chamber — within which is seen 
the spherical omphalic chamber, and the four spines. From the 
long axis of the shell, two of the delicate internal spines are 
shown suddenly increasing in massiveness. This condition is 
of very frequent occurrence. 
„ 17, 18, 19 are various beautiful and not uncommon kinds of radiate 
Astrommata of the Actinodinal series, in which the growth of 
the structures, (owing to the almost endless number of varietal 
and overlapping forms they assume) appears after the formation 
of the omphalic chamber and the one succeeding it, to diverge 
from the primary plan of growth in almost any direction. 
„ 20. A not uncommon Polycystine form, met with at the surface of 
tropical and subtropical seas, in which the shell is so imperfectly 
siliceous as to yield to pressure, and sometimes even to become 
distorted without fracture occurring. The material, however, 
is not at all highly refractive, as is the case with the Acantho- 
metrine spines. 
„ 21. Represents another not unfrequent Poly cystine shell from the same 
seas, but which is perfectly siliceous. Whether these two last 
represented forms are as closely allied as they appear to be, cannot 
be determined until the animal of the imperfectly siliceous form 
shall have been carefully examined in a recent state. 
,, 22. Pterocanium — one of the Monodinal series — a side view, showing 
the spike-like spine, the rudiment of which was formed at the 
apex of the omphalostype ; and has since then only increased in 
thickness and length. The omphalic chamber is seen supporting 
it, but, of course, very greatly extended outwards in dimensions 
and thickness. 
„ 23. The same as seen from the lower aspect. The aperture is more 
frequently almost as large as the diameter of the shell at this 
point, but occasionally, as in the specimen figured, the deposit 
of silex has gone on until the main opening is very little larger 
than the surrounding perforations in the shell. 
„ 24. Gives an enlarged view of a Monodinal Omphalostype , such as 
occurs in the form depicted in the last two figures. 
