ONCHIDIUM. 5 



from the other Pulrnotiifera, in the same manner as the 

 OpisthobrancMata do from the Prosobranchiata, so far, 

 at least, as the relative position of these organs is con- 

 cerned. And, moreover, it is worthy of remark, that 

 in Onchidium the anal orifice is distinct from that of 

 the lung, and that in Arion and Helix the excretory 

 apertures are within the pulmonary arch. The true 

 signification of these facts can scarcely at present be 

 determined ; more information is required respecting On- 

 chidium and its allies, to throw additional light upon 

 the subject. 1 ' 1 



