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V. Further Observations on the Iletamorphosis of Gasteropoda, and the Affinities of cer- 

 tain Genera, tvith an attempted Natural Distribution of the principal Families of the 

 Order. By John Denis Macdonald, Assistant Surgeon of H. 31. S. " Herald," 

 Captain H. M. Denham, B.N., F.B.S. Communicated btj Professor Huxley, 

 F.B.S., F.L.S. 



Read February 16, 1860. 



In oflfering to British Zoologists the following remarks on the metamorphosis of Gaste- 

 ropoda, I am anxious to premise that, as my present opportunities of consulting the 

 researches of others in this field are extremely scanty, it is quite possible that my inde- 

 pendent observations may appear to be but newly garbled statements of already published 

 facts. I am willing, however, to risk this, feeling that confirmation, which is the only 

 prop of simple assertion however truthful, must widen the basis of even accepted views. 

 If such confirmation be afforded by this paper, it will not be altogether unimportant ; 

 but if the facts are new, which they are likely to be, so much the better. 



"V^^'lien I first recognized a conformity in the dentition of numerous pelagic Gasteropods 

 with that of certain clearly defined families, the inference was natural enough, that those 

 little creatures formed a group in themselves, admitting of a like classification, though 

 still, as it were, of a representative value zoologically speaking. Another idea, however, 

 gained weight in my mind, namely, that they were merely the larval forms of genera bet- 

 ter known to us in their perfect or adult state. This view I have endeavoured to sustain, 

 in a former paper, with the evidence available at the time ; and however inconclusive 

 the arguments then adduced may have been, I am quite satisfied that the facts and 

 materials since obtained will place it beyond all reasonable doubt. 



While H. M. S. " Herald " lay at anchor on the leeward side of the Wreck* and Catof 

 Reefs, the tideway caused by the water sweeping over and arotmd those obstructions 

 brought many interesting objects to the towing net, including several genera of pelagic 

 Gasteropoda, with which we are more immediately concerned in the present paper. 



To enter at once into particulars, I first observed a little shell bearing the toothed and 

 sinuated lip of a Cheletropis, and appearing to be a new species of that genus ; but, having 

 been previously placed on my guard against this deceptive resemblance, which I detected 

 in another instance, I watched the movements of the occupant the more closely. I soon 

 found that the great activity of the little creature and the peculiar form of its foot, which 

 had a broad anterior margin, sloping sides, and a bifid posterior extremity, gave evidence 

 of its being the young of a species of Nassa ; and when its lingual dentition was examined 

 under the microscope, this proved to be the case. I may remark that the lingual strap of 

 Nassa is distinguished from that of Buccinum and Pisania by the absence of smaller den- 

 ticles between the two principal fangs of the pleurse, so characteristic of the Buccinidce. 



* Lat. 22° 10' 30". Long. 155° 30' 00". t Lat. 23° 15' 00". Long. 155° 35' 00". 



