CEPHALOPODA. 581 



tened processes called " labial," continued forwards from the inner 

 surface of the oral sheath, two of which are superior, posterior, and 

 external, the other two (k) are inferior, anterior, and more imme- 

 diately 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 papillas, and on each side of these a group of lamellae. Each 

 of the four labial processes is pierced by twelve canals, the orifices of 

 which project in the form of short tubes from the anterior margin, 

 and each canal contains a tentacle similar to, but smaller than, those 

 of the digitations. Thus the number of tentacles 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 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. Valen- 

 ciennes contained thirteen tentacles instead of twelve ; and some 

 variation is not surprising in the number of prehensile organs deve- 

 loped in such unwonted profusion in the Nautilus. A more marked 

 modification of the labial processes has been found in a male spe- 

 cimenf , which is probably sexual, and will be subsequently described. 

 The skeleton of the Nautilus consists of two parts, equally distinct 

 in their position, texture, and organic properties : the one is the 

 external chambered shell ; the other is a rudimental cartilaginous 

 cranium, which sends out processes for the attachment of the prin- 

 cipal muscular masses. The shell of the Nautilus consists of an 

 elongated sub-compressed cone, convoluted in close spiral whorls, 

 upon the same plane, so as to be perfectly symmetrical. In the full 

 grown mollusk, three-fourths of the shell from the commencement no 

 longer serve to lodge the animal, but have been partitioned off*, as 

 they have been progressively evacuated, into a number of chambers 

 (Jig. 214, b, b), increasing regularly and gradually in size from the 

 first to the last, or to the last but one. The open chamber (a, «), 

 which contains the animal, is much larger than the rest, slightly 

 curved, concave behind and on the ventral aspect, and divided into 

 two concavities on the dorsal aspect, by the projecting involuted 

 spire. At the sides are two slight impressions of the large lateral 

 muscles, and that of the connecting narrow cincture, and, posteriorly, 



* CCCLXXXIX, t CCCXC. 



r p 3 



