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 streligthened 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 umhilicatus. Lam. (fig. 26), and in the section of Nautilus pompilius 

 (fig. 27), as well as in the section of the fossil species Nautilus striaius (fig. 38) from 



Fig 26 — Nautilus umhhcatus, Lam. 



-Section of the shell of Nautilus Fig. 28. — Nautilus striatus, Sow. 

 pompilius, 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 hving 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 NautUus 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^ we shall 

 sometimes find the dwelling-chamber provided with a singular body, the Trigonellites^ 

 Park., or Aptyclius, 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 Ammonoida. 



