SUB-CLASS I TETRABRANCHIATA 509 
ciently buoyant to float itself and the animal when the head and tentacles are 
protruded in the act of swimming. Moseley! confirms the observations of 
Rumphius, but the animal he studied was drawn up by a dredge which had 
been dragged on the bottom at a depth of 300 fathoms. This individual 
swam in the manner described, but was not able to sink; and this was 
accounted for on the supposition that in rising from the bottom the sudden 
expansion and rarefication of the contents of the air-chambers had interfered 
with the action of the hydrostatic apparatus. 
Nothing has yet been ascertained regarding the mode of reproduction 
and development of the animal in Nautilus. The construction of the shell 
in this genus, however, renders it probable that in the youngest stage a 
perishable embryonal shell was formed, the presence of which is indicated by 
a scar or cicatrix on the apex of the initial chamber. Hyatt describes and 
figures a more or less wrinkled lump on the apex of several species of the 
Orthoceratidae, which he regards as an embryonal shell or protoconch ; and 
Clarke also figures one having a nearly perfect form. The former explains 
the absence of the protoconch in most genera and in the recent Nautilus by 
supposing it was usually membraneous or imperfectly calcified, and hence 
easily destroyed. 
As the animal continued to grow, it advanced forward by building out 
the edges of the aperture and secreted new septa at regular intervals, each 
one probably corresponding to a period of repose. A tubular prolongation of 
the base of the mantle was formed at each period of progress, and this 
remained behind in the first septal chamber and excreted the calcareous 
matter that built the last segment of the siphuncle. Each septum bends 
apically into a funnel around the origin of the siphon at the base of the 
mantle, and this is continuous with a calcareous but more loosely constructed 
and very porous wall that prolongs the tube begun by the funnel. This 
porous wall or sheath coats the funnel on its external surface in the air- 
chambers, but it continues alone apically beyond the funnel, and is inserted 
into the spreading trumpet-like opening of the next preceding funnel. The 
siphuncle is therefore a segmented, calcareous tube surrounding the siphon, 
each segment crossing only one septal chamber and consisting of a funnel and 
its connecting sheath.” 
In Nautilus the margin of the external opening or aperture is sinuous, the 
goncavities being the sinuses, the outward convexities the crests; and the 
single, median, concave bend on the venter is named the hyponomic sinus, 
because it indicates the position of the hyponome. In some fossil genera 
(Orthoceras) the aperture is often straight or simple (Fig. 1062); in others 
the lateral margins are produced in the form: of ear-like crests or lappets 
(Litwites, Ophidioceras); and in some forms they approximate more or less, 
forming contracted apertures. 
The closure of the aperture is never complete, and may take place through 
the inward growth of the lateral margins, as in Phragmoceras (Fig. 1087), 
forming a direct dorso-ventral slit, or from the venter and the sides, as in 
1 Moseley, H. N., Narrative of the Voyage of the Challenger, vol. I. p. 490.—Fischer, P., Manuel 
de Conchyliologie, p. 473, 1880-87.— Willey, A., In the Home of the Nautilus (Natural Science, 
vol. VI. p. 411), 1895. 
2 Brooks, H., On the Structure of the Siphon and Funnel in Nautilus pompilius (Proc. Boston 
Soc. Nat. Hist., vol. XXIII. p. 380), 1888.—Appelif, A., Die Schalen von Sepia, Spirula und 
Nautilus (Kon. Svensk. Vetensk. Akad. Handling., vol. XXV. No. 7), 1895. 
