311 



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 men turn of Natica. The little animal 

 creeps on its foot with great rapidity, appearing 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 

 Lymnsead forms a boat of its foot, it buoys up its tiny body and is 

 cast abroad on the face of the ocean. 



The paper was illustrated with coloured figures of most of the 

 objects described. 



II. On the Anatomy of Nautilus umbilicatus, compared with that 

 of Nautilus Pompilius" By JOHN DENIS MACDONALD, 

 Esq., R.N. Communicated by Sir W. BURNETT, K.C.B. 

 Received February 22, 1855. 



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

 1854, a recent specimen of Nautilus umbilicatus was picked upon 

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

 board, but was too much 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 N. 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 the 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. 



