Prof. Owen ow the Structure of the Pearly Nautilus. 307 



and it is on the corresponding part of the larger and fewer ten- 

 tacles of the Dibranchiate Cephalopods that the acetabula are 

 developed. The angle between the two outer finely annulated 

 surfaces (g) subsides near the end of the tentacle, which thus be- 

 comes flattened. 



" To the nineteen tentacula which are supported by the con- 

 fluent 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. 



" Both mandibles are horny, with their tips encased by dense 

 calcareous matter and their base implanted in the thick muscular 

 parietes of the mouth. They are immediately surrounded by a 

 circular fleshy lip with a plicated anterior border, external to which 

 there are four broad flattened processes continued forwards from 

 the inner surface of the oral sheath, two of which are superior, 

 posterior and external, the other two 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 lamellae. 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 anterior 

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

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

 tentacula with which the Pearly Nautilus is provided amounts to 

 not less than ninety, of which thirty-eight are termed digital, 

 four ophthalmic, and forty-eight labial. In the second specimen 

 of this rare molluscous animal, presented to the College of Sur- 

 geons by Captain Sir Edward Belcher, R.N., there was a slight 

 difference 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. Va- 

 lenciennes contained thirteen tentacles instead of twelve; and 

 some slight variation is not surprising in the number of pre- 

 hensile organs developed in such unwonted profusion in the 

 Nautilus. 



" The retraction of the tentacula is effected by longitudinal 

 muscular fibres, their elongation by transverse fibres. These are 

 not, however, disposed in circular or spiral series, so as to atte- 

 nuate and lengthen the tentacle by a general compression, but 

 present a more complex and beautiful disposition, by which they 

 diminish the transverse diameter without compressing the central 

 nerve. The transverse fibres (fig. 2. a) arise in numerous and 



Y2 



