CEPHALOPODA. 317 



on the corresponding part of the larger and fewer tentacles of the 

 dibranchiate cephalopods that the acetabula are developed. The 

 angle between the two outer finely annulated surfaces subsides near 

 the end of the tentacle, which thus becomes flattened. 



To the nineteen tentacula which are supported by the confluent 

 and free digitations on each side of the head, two others must be 

 added, which project from very short sheaths, one before, the other 

 behind, the eye ; the lateral transverse incisions are deeper in these 

 than in the digital tentacles. The eyes are about the size of hazel 

 nuts, and are attached each by a short peduncle to the side of the 

 head, behind the digitations, and a little below the margin of the 

 hood. The inferior surface of the oral sheath is excavated for the 

 lodgement of the infundibulum. 



The mouth is armed with two mandibles, shaped, as in other 

 cephalopods, like the beak of a parrot reversed, the lower mandible 

 i^fig' 129, I) overlapping and curving upwards beyond the upper 

 one iji). Both mandibles are horny, with their tips encased by dense 

 calcareous matter, and their base implanted in the thick muscular pa- 

 rietes of the mouth. 



They are immediately surrounded by a circular fleshy lip with a 

 plicated anterior border, external to which there are four broad flat- 

 tened processes continued forwards from the inner surface of the oral 

 sheath, two of which are superior, posterior, and external, the other 

 two ill) are inferior, anterior, and more immediately embracing the 

 mouth : the latter are connected together along their inferior margins 

 by a middle lobe, the inner surface of which supports a series of 

 longitudinal lamellae. On the inner surface of the oral sheath beneath 

 these processes there are two clusters of soft conical papillae, and on 

 each side of these a group of lamella?. Each of the four processes, 

 which I have called ' labial/ is pierced by twelve canals, the orifices 

 of which project in the form of short tubular processes from the an- 

 terior margin, and each canal contains a tentacle similar to, but some- 

 what smaller than those of, the digitations. Thus the number of tenta- 

 cula with which the pearly Nautilus is provided, amounts to not less than 

 ninety, of which thirty-eight may be termed digital;, four ophthalmic, 

 and forty-eight labial. In the second specimen of this rare molluscous 

 animal, presented to the college by Captain Sir Edward Belcher, 

 there was a slight diff'erence in number in the digital tentacula 

 of the two sides, nineteen being on the right, and seventeen on the 

 left side. The labial processes in the specimen of Nautilus described 

 by M. Valenciennes contained thirteen tentacles instead of twelve; and 

 some slight variation is not surprising in the number of prehensile 

 organs developed in such unwonted profusion in the Nautilus. 



