MUSEUM OF COMPARATIVE ZOOLOGY. 95 



closely examined, it is seen to be composed of two portions, the tumid 

 external sheath of looser granular texture, and the narrowing neck of 

 the funnel. Thus the tumid band is really, though so narrow, the repre- 

 sentative of the external sheath of the siphon in Nautilus Pompilius, and 

 performs the same function of uniting the siphonal funnels, and protect- 

 ing the continuous horny layer of the interior. In other Nautili, such as 

 Nautilus lineatus of the Jurassic, the siphonal funnel becomes a ring or 

 section of a cylinder, flaring or inclined outward both above and below 

 like an eyelet. 



Inside of the funnel and the sheath in Nautilus Pompilius is a 

 layer, which is also of a loose texture similar to the sheath, and inside 

 of this, the continuous dark corneous layer. The intermediate layer 

 has not been previously observed, and I have unsuccessfully endeav- 

 ored to find it in the siphon of the adult. The condition of the 

 specimen was such that I cannot now be sure that it is really a distinct 

 layer. 



The sheath in the adult is not simply of a loose granular texture, but 

 looks more like the rough surface of a sponge pierced with holes, 

 which are visible with an ordinary magnifier in desiccated specimens. 

 Externally, the sheath is covered by the lining membrane of the cham- 

 bers, and has a pearly aspect in many specimens. The thickness of the 

 sheath and its color vary greatly. In one specimen it was so exces- 

 sively thin and porous, even in the adult, that the color of the corneous 

 layer shone through. At an older stage, however, it assumed the usual 

 opaque aspect. Viewed from the interior, it is usually smoother, pearl v, 

 and shows broad bands, probably bands of growth. If so, the animal 

 must progress neither quickly from septum to septum, as supposed by 

 Owen, nor slowly and constantly, as described by Valenciennes, but 

 probably with many intermediate periods of arrest marked by the 

 deposition of these bands. 



The siphonal ccccum of Nautilus lineatus* occupies a different position 

 from that of Nautilus Pompilius. The coecum lies against the inner side 

 of the area of the cicatrix near the centre in Nautilus lineatus, and both 

 .-ides swell out under the first septum as on the dorsal side of the same 

 organ in Nautilus Pompilius. The siphon between the first and second 

 sepia makes a curve like the central portion of the letter S, first inclin- 

 ing ventrally, and then bending again towards the centre. The position 



* Plate IV, Fig. H>. 



