GENUS POLTSTOMELLA : CANAL-SYSTEM. 545 



spiral 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 

 lacunae, which are excavated in the solid nucleus. In Plate XVII. fig. 6, are shown 

 portions of two whorls (, a, and b, b) 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, d, c/, d) 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, 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, a, the orifices of the spiral canals transversely or obliquely 

 divided, and their connexion with the meridional canals b, 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 spiral 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, b; Plate XVIII. fig. 11, b, b) ; and that they are thus homologous 

 with the arches of the interseptal system of canals that connect together the spiral canals 

 of Operculina (^[ 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 2 ) 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 which 



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