OF NAUTILUS POMPILIUS. 23 



or anfractus of the revoluted shell. Hence the upper surface of this fold is excavated, 

 forming the exact counterpart of the shell's protuberance. Under that fold is a smaller 

 plate of nearly the same form, but adherent to the posterior declivous surface of the 

 hood, and only free at its circumference. This plate is of an aponeurotic texture and a 

 white colour : at both sides it is united to the dorsal fold, and below it seems to have 

 an intimate connection with the two side parts of the funnel, and indeed to be a conti- 

 nuation of those parts. The dorsal or superior part of the aponeurotic band, which 

 forms, as we have said already, the continuation of the oblong side-plate (fig. 1 g), is 

 here visible at g, g. Three small longitudinal bands or tendinous inscriptions {h, h, h) 

 seem to give some firmness to the dorsal part of the abdominal portion of the mantle. 

 Near the posterior end of this visceral sac, nearer however to the superior surface of it, 

 is the beginning of the siphon (j) ; it seems nearly superfluous to say that this siphon 

 is a tubular production of the visceral part of the mantle, protected by a calcareous 

 covering, and penetrating by the central perforation of the several septa in all the fol- 

 lowing compartments of the shell. 



At the inferior surface (fig. 3) a part of the funnel is visible in the middle of the digi- 

 tations of the head. The inferior face of those digitations is of a white colour, contrast- 

 ing with the brown and dark colour of the hood and of the superior surface of the digi- 

 tations which are nearest to it. The free inferior and anterior margin of the mantle 

 appears rounded and somewhat convex ; it conceals the basal part of the funnel and of 

 the appendages of the head. 



More instructive is an inferior view of the animal if the mantle has been removed or 

 reflected backwards ; in this manner the branchial cavity is visible (fig. 4). 



The two overlapping sides of the funnel form a striking particularity of the structure 

 of the Nautilus. It is interesting that the embryo in the dibranchiate group, as we 

 learn from Dr. Kolliker's observations', shows the funnel composed in the beginning 

 of two lateral separate parts. The embryonic condition in the dibranchiate Cephalo- 

 pods proves thus to be a persistent structure in the tetrabranchiate group. 



Between the basal part of the second pair of gills the anal aperture is visible. This 

 part has been misrepresented by Professor Valenciennes. It seems that a longitudinal fold 

 connecting the integuments of the viscera with the two large shell-muscles was dis- 

 rupted in his specimen, and that the author believed this to be the rectum. The oviduct 

 in this supine position is situated at the left side, before the anus, and terminates with 

 a transverse bilabiated and protuberant aperture or vulva. [Consequently, when the 

 animal is in its natural position in the shell, the termination of the oviduct lies at the 

 right side.] 



There are three little slits on each side at the roots of the branchiae. The first pair 

 of those apertures is situated at the anterior surface of the first branchia, near the pos- 

 terior margin of the large shell-muscle. Between the first and second branchiae are the 



' Entwickelungsgeschichte der Cephalopoden. Von Dr. A. KoUiker ; Zurich, 18'13, 4to, p. 41 &c. 



