MOLLUSCA — CEPHALOPODA. 
1.59 
away I'roiii it towards the centre, the space l)etween the 
two hecaine filled with calcite. These structures are very 
peculifir and often puzzling, especially when the inner tubes 
are found apart from the outer shells j in Huronici from 
the Silurian rocks of North America they look like fossil 
backbones, whence one species is named Iluronm vcrtc- 
hralis. Some of the shells of Actinoceras must have been 
immense, probably exceeding 10 teet (3 metres) in length, 
for there is exhibited the body-chamber of Actinoceras 
giganteuvi, which has a diameter of about 11 inches ; and a 
huge fragment, the whole of which appears to be septate, 
measures 2 feet 5 inches in length, the diameter oi the 
larger end being about 8 inches, that of the 
smaller about tX inches. This genus, which 
possibly occurs in the Cambrian, ranges 
through the Ordovician, Silurian, and 
Devonian to the Carboniferous Epoch. 
Many of these Palaeozoic ceplialopods 
are peculiar in that, after the shell has 
swollen out somewhat rapidly, perhaps to 
. accommodate the ripening eggs, it again 
contracts in diameter, and not only this but 
in some genera the edges of the shell close 
over the opening of the body-chamber, 
leaving only narrow apertures. Putcrioccras 
is one of the simpler types. The shell is 
pear-shaped, the earlier part being narrow 
and divided into chambers perforated by 
a siphuncle which is somewhat inflated 
between the septa ; the upper and larger 
portion contained tlie animal. The whole 
shell is usually slightly curved, and its 
aperture not closed in. Fine examples of this genus come 
from the Carboniferous Limestone of Ireland. It must have 
attained considerable dimensions, for an example of Poterio- 
ceras cortliformc from the red Carboniferous Sandstone of 
Closeburn, Dumfriesshire, is 9 inches long and 7^ inches in 
its greatest diameter. The shells of the Silurian Gomphocera- 
tidse are more egg-shaped and the opening is narrowed to a 
T-shape by the ingrowth of the margin (Fig. 89). The ends 
of the crosspiece and upright of the T are widened, and we 
may suppose that through the lower opening the funnel 
could eject its stream of water while some arms could 
emerge through the upper paired openings. Some allied 
Fig. 89. — Three- 
lobed aperture. 
lobed aperture. 
Gallery 
VII. 
Wall-case 
1 . 
