352 
THE VOYAGE OF H.M.S. CHALLENGER. 
condition the spire is quite regular and on a uniform plane, but sometimes the test 
exhibits a certain amount of lateral asymmetry, as seen in figs. 8 b., and 11. 
If the test be laid open horizontally, so as show the interior, it is found to consist of 
two to three convolutions of a gradually widening spire, regularly divided into chambers, 
as indicated by the sutural lines on the exterior. Fig. 12 represents a specimen so 
prepared, and fig. 14 a thin horizontal section viewed by transmitted light. The com- 
munication between the consecutive chambers is maintained by means of a curved slit- 
like orifice at the inner margin of each septum. 
But the most striking peculiarity revealed by the sectional view is the extraordinary 
development of the testaceous skeleton. Instead of a simple external wall of ordinary 
thickness and septa to correspond, the parietes are developed to such an extent that no 
cavities whatever are apparent in the earlier chambers, and in those of the final convolution 
the open spaces are relatively very small, so that in point of fact almost the entire test is 
occupied by the walls. 
As has been already stated, the walls are arenaceous, and composed of fine 
siliceous grains incorporated by a ferruginous cement. A thin layer on the exterior is 
solid and imperforate, as shown in fig. 13 ; but with this exception the entire substance, 
both of the outer wall and the septa, is cancellated, that is to say, it is perforated with 
tubular passages to such an extent as to present a porous spongy appearance. These 
passages are not mere accidental lacunae, but are regularly constructed, and the sand- 
grains surrounding them are arranged and cemented in orderly fashion. 
The only external indication of the interior structure of the test is to be found in the 
characters of the exposed septal face of the terminal segment of a growing specimen, 
such as that portrayed in fig. 11. In this figure the broad outer arch on the front of the 
last chamber represents the thickness of the shell-wall, and its nearly parallel tubulation 
is quite apparent, though better shown in the transparent section (fig. 14). The smaller 
area enclosed within this is the septum, the cancellated or tubulated structure of which 
appears externally in the form of irregularly disposed orifices. 
Fig. 16 is taken from a specimen from which a small portion of the external 
imperforate coating has been artificially removed, in order to show the ends of the 
tubular passages lying immediately beneath. Old and worn shells, and even some 
that are otherwise perfect, are occasionally found abraded in this way, and might 
easily lead to the supposition that the test in its normal state is perforate, which is not 
the case. 
Of the chemical composition of the test it need only be stated here that about 80 per 
cent, of the entire weight is silica, and nearly 9 per cent, peroxide of iron ; so that there 
can be little doubt that the latter body enters largely into the composition of the cement 
by means of which the sand-grains are incorporated. 
It may be added that sand-grains of large size are sometimes employed in the con- 
