GENUS TIIVOPOEUS: INTEENAL STETJCTUEE. 563 



distinct evidence, in sections of this species, of a spiral commencement, soon giving place 

 to an irregularly-cyclical growth ; and sometimes the first-formed portion (Plate XXI. 

 fig. 11, ft) bears so close a resemblance to the innermost part of the spire of Calcarina, 

 that in this earliest stage of their growth the two types could not be distinguished from 

 each other. Thus Tinoporm baculatus seems to bear the same relation to Calcarina, 

 that T. Icevis does (through Planorbulina) to the Eotaline type (^[ 217). The essential 

 difference between T. baculatus and T. Icevis consists in the possession by the former of 

 a " supplemental skeleton," which presents itself under two principal aspects. The piles 

 of chambers extending vertically from the equatorial plane towards the two surfaces 

 of the spheroid (Plate XXI. fig. 7) are partially separated by the interposition of pillars 

 of solid shell (fig. 8, a) ; and it is by the projection of the summits of these pillars (as 

 in Calcarina) that the tubercles of the surface are formed. The spines also, which form 

 the extremities of the radiating prolongations, belong to the same system ; and they are 

 shown, by sections of the Philippine type that pass in a favourable direction (fig. 6), to be 

 extended from a solid framework which begins to be formed even with the first convo- 

 lution, and which adds greatly to the thickness of the partitions between the chambers, 

 giving off a multitude of minute spines from their external surface (fig. 10, c, c). This 

 framework is penetrated by a canal-system, which not only forms passages through the 

 solid axis that is prolonged into the spines (fig. 10, a, a), but also extends itself into 

 the partitions between the chambers (fig. 9, b, b). The canal-system of the solid axis, 

 moreover, communicates freely with the cavities of the chambers that are adjacent to 

 it, as shown at fig. 10, b, b. These chambers are arranged around it with considerable 

 regularity, as is shown in fig. 5, which is a transverse section of the base of one of the 

 radiating prolongations, showing the solid axis with its radiating canals, surrounded by 

 three rows of chambers. It would seem as if, in the Polynesian variety of T. baculatus, 

 the material of the supplemental skeleton were appropriated rather to the formation of 

 the solid pillars than to that of a solid axis for the radiating prolongations ; the latter 

 being much less conspicuous than it is in the Philippine specimens, and sometimes 

 appearing to be deficient altogether except at their extremities. On account of the 

 variability of these differences, however, I cannot regard them as of any essential 

 value. 



222. If any further evidence had been required as to the essential relation between 

 the " canal-system " and the " supplemental skeleton," I think that it must be satisfac- 

 torily furnished by the comparison of the two species of Tinoporus now described. For 

 in T. Icevis it is obvious that the system of communications which exists between its 

 chambers is adequate for all the ordinary wants of an organism of this type, the structure 

 of which is uniform throughout. But when, as in T. baculatus, an additional framework 

 of solid walls is interposed in the midst of the building, for the support of the extensions 

 into which it is prolonged, a special system of passages, originating from the cavities of 

 the adjacent chambers, and extending throughout the solid framework, is provided for 

 its nutrition. 



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