THE VOYAGE OF H.M.S. CHALLENGER. 
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varieties of the genus cannot be separated by any sharp lines from some of the allied 
Nonionince , but the more typical forms present certain additional characters which are 
regarded as distinctively Polystomelline. The genus has been so frequently and so 
carefully described by previous writers, and its structure and affinities so fully treated, 
that it is only needful here to draw attention to the more important external features 
which are of service to the systematist. 
The test of Polystomella is, as a rule, of lenticular or discoidal form. In the weaker 
modifications ( e.g ., Polystomella striatopunctata ) the segments are more or less inflated, 
and the external furrows by which they are separated are bridged over at intervals by 
extensions of the inner margins of the segments, leaving rows of depressions or 
“ fossettes ” to mark the septal lines. These marginal extensions of the segments are 
called “ retral processes,” or in connection with their external shelly investment “ septal 
bridges,” and throughout a considerable section of the genus their presence to a greater 
or less extent is the only advance in structure upon that of the Nonioninw. 
In the more typical Polystomella the septa themselves, instead of being depressed, are 
limbate externally ; and the retral processes are numerous and sufficiently developed to 
form regular series of elevated transverse ridges, almost or completely connecting the 
septa of the consecutive chambers. Usually in these cases the septal bands are much 
thicker and more elevated than the transverse ridges [Polystomella crispa), but some- 
times they are equally developed, and form a surface-reticulation of uniform height 
( Polystomella verriculata). 
The aperture of the feebler varieties consists of an arched cleft at the inner margin 
of the final segment, scarcely differing from that of Nonionina ; but in other forms the 
superior edge or lip is denticulated, so as to subdivide the opening to a greater or less 
degree by cross bars, and in Polystomella crispa it consists of a Y-shaped row of 
rounded orifices, which may be regarded as the typical condition. 
As already stated, the test of Polystomella in its normal aspect is equilateral and 
biconvex, though individual specimens exhibiting a certain amount of bilateral asym- 
metry are by no means uncommon. There exists, however, a small group of forms, 
named by d’Orbigny Faujasina, in which the shell is plano-coqvex and the arrangement 
of the spire like that of Truncatulina lobatula, the whole of the convolutions being 
visible on the flat side of the test, the final convolution only on the convex face. 
The more highly organised species of Polystomella have a supplemental skeleton, 
provided with a somewhat complex system of canals. The precise form and arrangement 
of the canals differ materially in the different species, though (broadly speaking) referrible 
to one general plan. Two “ spiral canals,” one on each face of the test, follow the 
umbilical edges of the spiral lamina ; these are connected by “ meridional ” canals lying in 
the septal depressions between the chambers ; and the latter communicate with the 
exterior by short tubes, single or forked, which open upon the septal lines. In the 
