20G MR. MACDONALD ON THE HOMOLOGIES OF THE SO-CALLED 



There is no example of a Conchifer iu which the right-mantle 

 margin does not blend with the left, at least in the region of the 

 hinge ; while, on the other hand, there is no instance amongst Gas- 

 teropods in which the margin of the right or " opercular mantle'" 

 coalesces with that of the left or " shell mantle" at any point. 

 Here, then, is a simple feature distinguishing two Orders of Mol- 

 lusca, arising out of the determination of the homology of the uni- 

 valve and opercidum of the Gasteropod with the bivalve of the 

 Conchifer. 



Preserving their homology with the two adductors of " dimyary 

 ConcMfera" and perhaps also combining the representatives of 

 those fibres which serve to withdraw the foot, the great retractor 

 muscles of Gasteropoda generally consist of two principal fasciculi ; 

 and even in the shell-protected embryo of those species which are 

 destitute of a shell in the mature state (e. g. Eolis) the temporary 

 retractor is composed of two distinct bundles. The double nature 

 of this muscle is usually more apparent at the opercular extremity 

 than at the opposite end, as in the beautiful genus Atlanta for 

 example, in which the retractor muscle, arising by a broad oblique 

 attachment extending some little distance upon the right or upper 

 wall of the tube, is inserted into the operculum by two distinct 

 fleshy slips (the great " pedal sinus " intervening), while certain 

 portions diverge into the vertical fin and sucker-disc. A still 

 better example occurs in the genus Neritina, in which a wide space 

 intervenes between the two retractors. The inner extremity of 

 these muscles scarcely rises above the level of the general surface 

 of the mantle, presenting a broad smooth facet on either side of the 

 body. Indeed the corresponding muscular impressions in the 

 bowl of Navicella shadow forth most unequivocally those of the 

 anterior and posterior adductors in Dimyary bivalves. The cor- 

 rectness of this view is supported by the fact that the fibres com- 

 posing the lateral muscles may be traced through the operculige- 

 rous lobe towards the operculum, into which they are separately 

 inserted. The articulation of the operculum of Neritina with the 

 columella of the shell is retrospective, as it were, of the state of the 

 typical bivalve, while in other respects the higher type of the 

 Gasteropod is more strikingly developed. 



The theoretical conversion of the left valve of a Conchifer into a 

 spiral dextral shell, with a rudimentary and modified right valve 

 forming a suitable operculum, requires no great stretch of the 

 imagination to conceive. The retractor muscles of Gasteropoda 

 passing between the shell and the opei'culum, take vip a longi- 



