TETRABRANCHIATA 09 



ciently buoyant to float itself and the animal when the head and tentacles are 

 protruded in the act of swimming. Moseley 1 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 

 swain 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.' 2 



In Nautilus the margin of the external opening or aperture is sinuous, the 

 concavities 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 

 (Lituites, 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. X., Narrative of the Voyage of the Challenger, vol. I. p. 490. Fischer, P., Manuel 

 de Couchyliologie, 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), l88S.Appelof, A., Die Schalen von Sepia, Spirula und 

 Nautilus (Kon. Svensk. Vetensk. Akad. Handling., vol. XXV. No. 7), 1895. 



