204' ME. ilACDONALD ON THE HOMOLOGIES OF THE SO-CALLED 



shallower, more oblique suture ; tlie pillar-lip is more dilated and 

 prominent at the base ; and lastly, the peristome is not obtuse or 

 thickened and coloured. From all the other allied Madeiran 

 species it differs in its cylindric shape and short ovate aperture, 

 simply acute, and not narrowed or acuminate at top. It is also a 

 considerably larger shell than A. gracilis and A. Leacociana, Lowe, 

 to which, amongst these other species, after A. producta, Lowe, it 

 most approaches ; and from A. producta, Lowe, with which in shape 

 and size it best agrees, it is abundantly distinct by the form and 

 proportionate size of the aperture, besides the much less prominence 

 of the piUar at its base. Prom A. folUculus (Gron.) it differs in 

 the narrow turreted-cylindric shape, the short ovate aperture, and 

 the perfectly even and regular volutions (v^dthout any turgidness 

 in the penultimate volution) of the spire. 



On the Homologies of the so-caUed Univalve Shell and its Oper- 

 culum. By JoHK Denis Macdonald, E.N., F.E.S., T.L.S. 

 Communicated by Prof. Huxlet, F.E.S. 

 [Eead February 16th, I860.] 

 Adanson believed that the univalve spiral shell of the Oasteropod 

 was the homologue of the sinistral valve of the Conchifer, and 

 also that the dextral valve of the latter was represented by the 

 operculum of the former. Dr. Gray appears to support this view ; 

 but as it has met with much opposition from recent writers, who 

 have substituted far more unnatural and fanciful theories, I 

 have been induced to draw up the present paper in support of the 

 doctrine, which I believe to be sound and philosophical, and sug- 

 gestive of other remarkable homologies which must be ever con- 

 cealed from our view while this primary barrier is permitted to 

 remain. In dealing with this subject, it wiQ be more convenient 

 to beg the question in the outset, and sustain it as well as all the 

 subsequent reasoning may be capable. 



If Adanson's position, as above given, be right, it will be ad- 

 mitted that the left valve, so to speak, of the Oasteropod is more 

 generally that which receives the body of the animal ; but in those 

 examples exhibiting a transposition of the viscera, and therefore a 

 sinistraUy spiral shell, the presumption is that the right valve 

 (normally the operculum) envelopes the animal, while the left 

 valve forms the operculum. As far as I have been able to discover, 

 these are the only cases in which a dextrally spiral operculum 

 closes the aperture of a sinistral shell ; for in those instances of 



