176 
THE LIAS AMMONITES. 
The head is provided with a large ligamento-muscular plate, or flattened disk, formed 
by the junction and dilatation of a pair of dorsal tentacles, which, besides acting as a 
defence to the opening of the shell, serves probably for creeping along the bed of the sea, 
like the foot of a Gasteropod, as the Nautilus has neither fins nor natatory organs, and 
moves through the ocean by the ejection of the sea water out of its locomotory funnel. 
The mandibles are strengthened by a dense external calcareous coating, which forms a 
dentated margin on the jaws. 
The shell is involute or discoidal, few-whorled, and many-chambered, as seen in the 
shell of Nautilus umbilicatus, Lam. (fig. 26), and in the section of Nautilus pompilius 
(fig. 27), as well as in the section of the fossil species Nautilus striatus (fig. 28) from 
Fig. 26. — Nautilus umbilicatus. Lam. Fig. 27. — Section of the shell of Nautilus Fig. 28. — Nautilus striatus, Sow. 
pornpilius, Linn. Section of a fossil Nautilus, showing the 
position of the siphuncle. 
the Lias Eormation. The septa are here seen to be concave towards the aperture, and 
convex towards the spire ; near the centre of each septum a short funnel-shaped process 
projects backwards (fig. 27) ; around this the membranous siphuncular tube is firmly 
attached, by which a continuous pipe extends from the pericardium to the first chamber 
of this polythalamous shell (fig. 24). The outer laminae of the shell are porcellaneous, 
and the inner nacreous ; and the Chinese and others carve a variety of patterns out 
of the opaque porcellaneous portion of the Nautilus shell, which are relieved by the 
pearly layer beneath, which forms the background of the object. The living Nautili have 
the external surface of the shell smooth, but among the fossil forms many of the species 
are variously sculptured. The aperture of the shell in the Pearly Nautilus is closed by a 
disc or hood formed by the union of the two dorsal arms which are homologous to the 
shell-secreting "sails" of the Argonaut (figs. 15, 16). In the Ammonitid.e we shall 
sometimes find the dwelling-chamber provided with a singular body, the Trigonellites^ 
Park., or Aptychus, Von Meyer, which played an important part in the organic func- 
tions of this large extinct group of tetrabranchiate Cephalopods. 
This order is divided into two sub-orders, the Nautiloida and the Ammo]S[oida. 
