THE PAL.EOZOIC SEED LAGENOSTOMA OVOIDEs. 469 
Taking first, broadly, the thick-walled (а) and the thin-walled (b, е, d) 
types above, it might be suggested that the latter corresponds to the original 
structure, and that the opposite appearance is produced by a post-mortem 
swelling of the walls. And this view might be considered to gain support 
from the fact that the best-preserved preparation (R. 22) is of the thin-walled 
type. But were this the case, the thick walls would scarcely be so heavily 
carbonized as they are, and might be expected to present the appearance 
already described for the more or less swollen walls of the epidermis 
(p. 466). Far more convincing evidence that the thin-walled is not the 
original type is, however, afforded by careful comparative observation of the 
character of the walls and contents. This has led to the conclusion that the 
fibres were originally thick-walled, and that the opposite appearance is pro- 
duced by the peeling of the walls in layers, the end process ot which is shown 
in В. 22 (РІ. 99. fig. 2), where many of the walls may be seen to be split, in 
some cases a complete inner shell being formed. А pretty t 'ansitional case 
occurs in a broken bit of the shell found in preparation В. 36 (РІ. 24. fig. 24), 
where the thick walls are seen to be laminated, which, according to this view, 
would be the first stage in the splitting and absorption of the walls. In some 
cells the walls are just beginning to split. 
We have now to explain, as far as possible, types (c) and (d) as representing 
originally thick-walled cells. The dark masses in the centre of the cells in 
РІ. 22. fig. La were spoken of as contents; but this is probably only true 
descriptively as they stand, for these masses almost certainly do not represent 
the original contents of the cells, but rather the fused, disintegrated peelings 
of the walls. This is well shown by the series of transitional cases drawn in 
Pl. 22. fig. 1 b, where the lowermost cell is seen to be very similar to those 
shown in fig. la as typical for this preservation. In the uppermost cell, 
however, the dark “contents” are in the form of a thick ring with a large 
central space, and are most easily explained as being derived from the wall 
and as equivalent to several laminze of the cells in P1. 24. fig. 24. The middle 
cell forms a transition from one condition to the other, the “ contents ? being 
contracted till only a small lumen is left, which in the lowest cell has entirely 
disappeared. The difference between this type and that in which the wall 
shows distinct layers (0) is probably that in the former case decay had gone 
a step further before the structure was fixed by mineralization. The last 
type (d) is more difficult to harmonize with the view here advanced. It is 
possibly a variety of (c), but it seems more likely that the very dark central 
spots do really represent the original cell-contents. This is supported by the 
fact that some of the laminated cells in the fragment of R. 36 show similar 
small black central spots, which it is difficult to interpret as other than 
representing the original protoplasts. If this is so, we must suppose that the 
inner layers of the wall have undergone a further stage of decomposition 
prior to fossilization. 
