107 



smooth and polished, shows that these calcareous papillae are secon- 

 dary growths deposited after the gelatinous segment has surrounded 

 itself with a calcareous covering, thus proving that the creature has the 

 power of strengthening its habitation by adding pillars and buttresses, 

 which thicken its walls. At 1, d, where the section has cut the seg- 

 ments across near the union of the septum with the lateral parietes 

 of the cell, we see that these papillae become largely developed, 

 shooting off from the septum like so many buttresses flanking the 

 sides of an arched building. Since very few foramina pass through 

 the external walls of the shell where these septa, with their flanking 

 appendages, are united to them, we can trace the direction assumed 

 by these latter portions, even on the exterior of the shell : they evi- 

 dently produce the radiating lines which, by their translucent aspect, 

 add so much to the beauty of its exterior. 



As we trace the direction of the spiral outline, beginning at its 

 external extremity, we find that the parietes become thickened as 

 we proceed. In the central portion this appearance is partly caused 

 by the oblique direction in which the section traverses it ; but 

 independent of this, there is, up to a certain point, a real increase of 

 thickness, as is proved by a glance at the vertical section (fig. 2). 

 Thus, though the section has traversed the organism in nearly the 

 same plane at 1, e, and at 1, a, we observe that the thickness of the 

 . shell is much greater at the latter point than at .the former. The 

 probable explanation of this increase of thickness will be given after 

 describing the next specimen. 



Fig. 2 represents a section which I made at right angles to the 

 former one, in the direction of a line drawn between the points fig. 

 1, a and a'. From this specimen we see that the parietes of the 

 shell consist of a great number of minute parallel lamellae, traversed 

 in certain parts by long and well-defined tubes for the transmission 

 of pseudopodia ; whilst in other portions these tubes are wanting. 

 The latter is especially the case in the central portion (2, b), corre- 

 sponding with the umbilical region, or columella of a spiral mol- 

 lusk, and at the angular external margin of each segment (2, a and c). 

 In an occasional instance only (2, d), a few tubes traverse the latter 

 portion of the organism. 



In this section we again see the small calcareous papillae (2, e) 

 studding the floor of each chamber, whilst the internal surface of the 

 outer wall is free from them, excepting where the oblique direction 

 assumed by the septa brings the two together as at 2, / and /. 



