788 PROF. G. C. BOUHNE ON THE 



a lessei" degree, Orobophana, when they withdraw themselves into 

 their shells, do not contract themselves as much as the "West- 

 Indian species. The sole of the foot in these Poljaiesian genera 

 is longer and narrower than in the West-Indian genera, the 

 columellar muscles longer and inserted further back from the 

 mouth of the shell. There is therefore more ample room for the 

 head and foot in the last whorl of the shell, and when the animal 

 retracts itself the foot is scarcely at all contracted, but simply 

 slides back with its sole applied to the oviter side of the shell till 

 the operculum borne on the broad opercular lobe closes the 

 aperture. Specimens of these genera, when extracted from their 

 shells, do not present the deformed ajJi^earance of an Alcadia or 

 an Eutrochatella, the pedal nerve-cords are not turned forward 

 with their morphological surfaces reversed, and if the animals 

 were only a little larger they would be much easier to dissect 

 than their American congeners. It is possible, and even probable, 

 that these different modes of retreating into the shell, which are 

 themselves dependent on the varying length and points of 

 insertion of the columellar muscles, are correlated with the 

 different forms of operculum upon which "Wagner has founded 

 his system. At any rate, they are consistent with it, but I have 

 not been able to follow out this problem in detail. 



To return to the nervous system. The opei'cular nerves must 

 not be mistaken for the parietal nerves correctly described and 

 figured b}^ Bouvier and labelled cV , e . The parietal nerves 

 (PI. XXXIX, fig. 44, n.jKir.) are much more slender than the 

 opercular nerves and originate, as shown in fig. 44, from the 

 pleural centres, between the great pallio-columellar nerves and 

 the cerebro-pleural connectives. They pass to the muscular walls 

 of the head behind the tentacles. From the venti'al side of the 

 swollen anterior ends of the pedal cords, in the same cross-section 

 as the opercular nerves, a rather stout pair of nei'ves originates 

 near the middle line ; these nerves, which ai-e shown in section 

 in fig. 46 (PI. XXXVIII.), pass to the pedal gland and appear to 

 be specially connected with that organ. 



As regards the subintestinal nerve and its distribution, I am 

 unable to bring my observations into agreement with those of 

 Bouvier. The short nerve connecting the subintestinal with the 

 left pallio-columellar nerve-trunk does not appear to be a constant 

 feature. I have found such a connective in a single specimen of 

 Alcadia jjalliata, and in that one instance it is much closer to the 

 pleuro-pedal centres than is shown by Bouvier. But I can find 

 no trace of it in any other specimen that I have examined. I am 

 unable to find any trace of the visceral nerve labelled / in 

 Bouvier's figure, which he describes as given ofi" from the left side 

 of the subintestinal at some distance from the origin of the latter, 

 and in general my observations on the subintestinal and visceral 

 nerves difier so much from his that a detailed account is necessary. 

 The figure illustrating this account (PI. XXXIX. fig. 43) is founded 

 upon dissections, and the ultimate ramifications of the principal 



