TWO GIGANTIC TYPES OP ARENACEOUS PORAMIN IEEE A . 
747 
be distinguished, unless magnified, in the general labyrinthic system. This distinction 
of large and small subdivisions is one of degree only, and is not nearly so striking 
under the microscope as might have been expected from the appearances presented to 
the unassisted eye ; but it nevertheless does exist, and is a means to an end. 
40. The ‘accessory skeleton’ in Loftusici may be regarded as the homologue of the 
labyrinthic lamellse in Parkeria-, although there are important differences, the precise 
significance of which it is impossible to explain with our present limited knowledge of 
the relation of the two types. The most remarkable of these is that in Loftusia the 
labyrinthic portions take their rise from the inner surface of the primary spiral lamina, 
and are directed inwards, that is, towards the central axis ; whilst in Parkeria the order is 
reversed, the corresponding structures presenting their free ends towards the periphery. 
Again, the ‘ radial tubes ’ which complicate the labyrinthic system in Parkeria , have no 
precise analogy to any portion of the accessory skeleton of Loftusia ; the nearest approach 
to anything of the sort being the lines of tubular communication between the septa of 
the individual layers, at the ends of the shell, where the layer is thickest ; but here, as 
in other parts, the spiral lamina cuts off direct communication between the layers. The 
office fulfilled by the accessory skeleton in Loftusia is, I conceive, simply that of a 
support to the primary lamina, imparting the necessary solidity to the organism. The 
subdivision of the chambers into chamberlets seems to be an accidental circumstance, 
and has but little bearing on the general economy of the animal. 
41. In considering its fitness for this purpose, the various external relations of the 
organism must be borne in mind. It is manifest that the delicate calcareous lamina, 
described as the 4 primary wall,’ would, both from its contour and extreme tenuity, be 
utterly insufficient of itself to protect a mass of sarcode three inches long and one inch 
in diameter, or to impart that rigidity which, judging from other examples, is necessary 
to animals of its class. 
42. The layer immediately within the 4 primary wall ’ adds greatly to its strength, not 
only from the additional thickness it imparts, but also from the connexion its septal pro- 
longations establish between the successive whorls. This portion, however, does not 
represent a solid mass ; and the septal portions are further weakened by irregular perfo- 
rations for the stoloniferous tubes connecting the sarcode of the adjacent chambers. The 
columnar extensions of the shell-substance provide direct vertical support ; and their 
distribution on the plan described is that likely to ensure the maximum of strength 
combined with economy of material. A longitudinal section whose direction nearly 
coincides with the long axis of the chambers (Plate LXXX. fig. 4), shows the primary 
walls as parallel lines, and the septa (s, s) as slightly oblique bands diverging in the most 
gradual way, and eventually connecting one wall with the other. In the exaggerated 
view so obtained, the columnar supports (ts, ts ) appear at regular intervals; and 
throughout the long narrow ends of the chambers they are very close and massive in 
proportion to the space left for sarcode. As the chamber widens, they diminish in fre- 
quency and proportionate dimensions. 
5 G 2 
