Mr. Macdonald un the Anatomy 0/ Nautilus umbilicatus. 301 



the remainder of the animal and the shell. The shoe-upper, as it 

 were, presents two rounded lateral lobes which lie over the anterior 

 part of the shell, like the mentum of Nutica. The little animal 

 creeps on its foot with great rapidity, ajjjjcaring rather to slide along 

 than progress by a vermicular movement, and by spreading out and 

 hollowing this organ at the surface of the water, as a freshwater 

 Lymnaead forms a bo-at of its foot, it buoys up its tiny body and is 

 cast abroad on the face of the ocean. 



" On the Anatomy of Nautilus umbilicatus, compared with that of 

 Nmitilus Fompilius." By John Denis Macdonald, Escj., R.N. 



During a visit of H.M.S.V. 'Torch' to the Isle of Pines in July 

 1854, a recent specimen oi Nautilus umbilicatus was picked upon 

 the outer reef off Observatory Island. It was alive when brought on 

 board, but was too mucli exhausted to exhibit active movements. 

 Part of the hood appeared to have been eaten away behind by some 

 predaceous enemy, but in other respects the animal was perfect. 



The body when retracted lay more deeply in the shell than that 

 of A''. Pompilius, so that no part was visible in a lateral view, and on 

 account of the great depth of the chamber of occupation the orifice 

 of the siphuncle in the last septum could not be seen when the soft 

 parts were removed. As to this difference, however, the author ob- 

 serves that it may depend on the time elapsed since the formation of 

 the last partition. 



Apart from tlie shells, the author finds a close resemblance be- 

 tween the corresponding parts of the two species. 



The specimen of N. umbilicatus examined proved to be a female ; 

 a fact which may serve to modify the views of those who, adopting 

 the speculations of D'Orbigny on the sexes of the Ammonites as in- 

 dicated by the characters of their shells, apply them also to the 

 several kinds of Nautili known. 



The body of N. umbilicatus is larger and more elongated than 

 that of A'^. Pompilius as it occurs in the South Seas, although the 

 specimens of the latter species brought from the Chinese Seas much 

 exceed both in size. In the N. umbilicatus, the longitudinal lamellae 

 on the median lobe of the external labial processes are divided by a 

 wide groove into two lateral sets, and the corresponding lamellae be- 

 tween the internal labial processes are about seventeen in number 

 and of considerable thickness. In N. Pompilius, the latter lamellae are 

 much thinner and more numerous, and the lateral sets of the former 

 are united together in the median line, commencing anteriorly with 

 an azygos transverse lamina. In both kinds, however, the corre- 

 sponding tentacula may be distinctly traced out, with only such 

 minor differences as might be expected to occur in different speci- 

 mens of either separately ; the digital, labial and ocular groups 

 agreeing sufficiently both as to number and character in the two 

 cases, considering the liability of these parts to slight modifications, 

 from arrest of development or redundance, in the same species. 



Referring to foimer observations of his own on the eye of A^ Pom- 

 pilius, the author observes that they closely apply to A'^. umbilicatus, 



