GENUS POLYSTOMELLA : — CANAL-SYSTEM. 
545 
spii'al canal giving off the meridional canals, and these again sending off their diverging 
branches. Towards its centre, the spiral canal communicates with an irregular set of 
lamnce, which are excavated in the solid nucleus. In Plate XVII. fig. 6, are shown 
portions of two whorls [a, a, and h) of the spiral canal, as shown in a section passing 
near the lateral surface of the outer convolution; proceeding from the outer side of 
these, in like manner, we see the meridional canals (c, c, c', c', c') with their first pairs 
of diverging branches (see also fig. 12) ; whilst another set of canals (d, d, d) is seen to 
proceed from the inner side of the spiral canals, tending more or less obliquely towards 
the lateral surface ; and the portion of the section which passes through the solid cal- 
careous nucleus is seen to be perforated by numerous apertures (^, e) of corresponding 
diameter, disposed at pretty regular intervals. The relation of these to the canal- 
system is clearly evidenced by vertical sections, such as those represented in figs. 2 and 
11 ; in which we see at a, «, the orifices of the spiral canals transversely or obliquely 
dmded, and their connexion with the meridional canals b ; and which further show 
that the solid calcareous nucleus is itself traversed by straight canals, c, c, c, which 
spring from the successive convolutions of the spiral canal, and pass directly, without 
branching or inosculation, to the external surface. That this remarkable portion of the 
canal-system does not fully show itself in the “ casts ” represented in Plate XVIII. 
fig. 12, is easily understood, when it is remembered that the whole substance traversed 
by the straight canals having been removed, their long and slender casts would be left 
entirely without support ; and the points at which these have been broken off from the 
cast of the spu’al canal are in fact to be seen on a careful examination, as there 
represented. 
189. It is shown by the comparison of vertical and horizontal sections of the shell 
with fragments obtained by fracture (Plate XVII. figs. 2, 7, 10, Plate XVIII. fig. 11), 
that the meridional canals are in reality spaces left by the divergence of the two layers 
of which each septum is composed, in the immediate neighbourhood of its junction 
with the spiral lamina which constitutes the external wall of the chamber (Plate XVII. 
fig. 7, c, and fig. 10, h] Plate XVIII. fig. 11, 5, h) ; and that they are thus homologous 
with the arches of the interseptal system of canals that connect together the spiral canals 
of OjiercuUna (^ 157), — presenting, however, a much greater uniformity and constancy 
in their disposition. The diverging branches given off from these (Plate XVII. 
fig. 8, d, d\ d^) consequently pass at once into the spiral lamina, through which they 
run obliquely towards the external surface of the convolution, usually increasing in 
diameter as they proceed. The divergence of the branches of each meridional canal 
causes those proceeding from adjacent canals to approach one another; and when the 
spiral lamina has attained its full development, they not unfrequently open at its sur- 
face into the same depression, this being midway between the septa from which they 
respectively sprang ; and it appears to be from the correspondence of these junctions 
with the intervals between the segments of the succeeding whorls (as seen at e, e'), that 
the alternating arrangement of the chambers of consecutive whorls arises, of w'hich 
MDCCCLX. 4 c 
