24 Tlie American (icologist. July, i»9^ 
which have lon^" been recognized to be the means of fission. 
Neither can the chain-like arrangement of the corallites be 
used to compare this genus with Halysites. 
As a third differential character between ChcEtctcs and Te- 
tradhun the presence of a double wall has been described by 
Nicholson. Neumayr remarks that this observation has not 
yet been verified. In fact, only a very small percentage of 
sections of coralla, in the writer's collection, sliow any traces 
of a double wall at all. As the figures i, 3 and 4, well illus- 
trate, the walls are formed by closely packed, transparent cal- 
cite crystals, the outlines of which are indicated in the figures. 
Sometimes one crystal extends throughout the entire width of 
the wall. The crystalline nature and the perfect lack of struc- 
ture in these walls is the more striking, as the brachiopod.-^ 
and bryozoans in the same slides exhibit the finest details of 
their structure. It, then, would seem, that the walls of Tc- 
traduim were homogeneous and simple. But as all walls, with 
the exception of the common outer wall, have been secreted 
in folds of the ectoderm and hence, appear as plications of the 
outer wall (see figs, i and 2), it is evident that they must have 
been double originally. A careful scrutiny of the walls re- 
vealed indeed the fact that some sections (cf. fig. 8) possess a 
dark line extending a short way from the exterior into the 
plications, thus demonstrating the amalgamated nature of the 
walls. There have also been found a few specimens (see figs. 
16 and 1 6a), in which a dark median line passes continuously 
through the walls. It follows a zig-zag course on accovmt of 
the secondary plications which branch off on alternate sides 
of the primary ones. A stronger enlargement of the walls 
shows that the dark median line is caused by the presence of 
the same organic coloring matter which renders the matrix 
opaque and which here is deposited in the calcite crystals (cf. 
fig. I 6a)*. 
The conclusion to be drawn from these observations is 
that the walls originally were double, that, however, with very 
few exceptions, a complete amalgamation took place already 
in the earliest stages as proved by sections of excellently pre- 
*These dark median lines must not be confounded with the fracture- 
lines running quite regularly in the walls in such sections which have 
been ground with too coarse emery. 
