192 Mr. H. J. Carter on the Polytremata. 



circumferential layers, but rest on the convexities of the latter 

 as often as on any other part — that is, so irregularly as to 

 make it impossible to connect them into a continued line of 

 support (fig. 3, a, h) even like the " solid pillars " of Orhitoides 

 &c., which are not only solid but continuous from the circum- 

 ference to the centre of the test. Thus the structure of Poly- 

 trema miniacewn is wholly cancellous, and without other support 

 than that which the peculiar arrangement and form of the fora- 

 minated layers and their hollow pillars give to the whole mass. 

 It is, in fact, just what Dr. Carpenter has described in "ParZ^ma" 

 withoutthe "labyrinthic structure" (Phil. Trans. 1870, vol. clvii. 

 pt. 2, p. 721) ; while if the " parallel columnar or tubular pro- 

 cesses springing from the internal surface of the spiral lamina " 

 of Loftusia [ih. p. 745, pi. 79. figs. 1 & 2) are to be considered 

 equal to the foraminated subconcentric walls of Polijtrema 

 miniaceum, the radial columns of the latter, whose cavities are 

 continuous with this foraminated structure, would appear to be 

 almost identical with those of Loftusia. 



Be this as it may, in the early part of the life of Polytrema 

 miniaceum (that is, during the time the test is being con- 

 structed), the object appears to be to combine the greatest 

 amount of strength with the least quantity of material, and 

 thus the radial processes are hollow ; while only in the after 

 part of the life of the Polytrema they become consolidated, 

 and the tubulation of the concentric layers obliterated, if not 

 removed altogether, except oa the surface^ as before stated. 



I also now observe, in a mounted thin horizontal section of 

 Alveolina meandrina which I retained when the rest of 

 the specimens of this fossil were given to the Geological 

 Society of London in 1863 (originally coming from the valley 

 of Kelat, not very far from the Bakhtiyari Mountains, in Persia, 

 where the late Mr. Loftus found "Loffusia^^), that the former 

 presents a cavernous or cancellated structure almost identical 

 with that of " Loftusia ^^ and Polytrema miniaceum, Avhich 

 structure, in my short description of Alveolina meandrina 

 ('Annals,' 1861, vol. viii. p. 381, pi. xvii. fig. 4,/), I had 

 erroneously regarded and represented (fig. 4,/, 2 & 3) as the 

 " canal-system " in this species, while it now proves to be 

 what is above stated and no " canal-system " at all, as the 

 latter, if there had been any, would be recognized by its tubular 

 form within the cavernous structure. My " marginal reticu- 

 lation" (fig. 4,/, 2) and ''vertical canals" (fig. 4,/, 3) would 

 thus be analogous to corresponding parts in Polytrema minia- 

 ceum^ Parkeria, and Loftusia, the pillars being mere sup- 

 ports and not indications of the limits of the chambers, which 

 are otherwise marked by the successive curving inwards of the 



