71 J. D. MACDONALD ON THE CLASSIFICATION OF THE GASTEROPODA. 
We now know enough of the distribution of otoliths and otoconia in the Molluscous 
Orders to perceive that there is a definite purpose in the bestowment of the one or the 
other, in particular cases ; and as this purpose is never the subject of caprice, but perma- 
nent, the resulting characters must be always significant, and therefore available in classi- 
fication. 
The division of the lingual dentition into straps and pavements, as previously defined, 
is not altogether free from imperfection, although it is most satisfactorily available in by 
far the greater number of genera. Indeed, it is only amongst the Opisthobrcmchiata that 
any difficulty in this respect presents itself. Can anything, for example, be more diver- 
sified than the characters of the lingual dentition in the Bullidce of authors ? No less than 
four distinct types are distinguishable in this family, thus : 
Central series absent. 
A 
Central series present. 
A. 
Laterals 1 or 2, Laterals more Laterals more Laterals 1 or 2, 
uncinate, numerous, numerous, the third rudimentary, 
as in as in as in as in 
Scaphander Bullina (Risso)* Bulla (numerous species) Bulla ampulla 
and and and and 
Bullosa. Aplustrum. Cylindrella. Amphisphyra. 
(Philinidae of Gray.) (Bullinid 
) 
(Bullidse of Gray.) (Occurring respectively m 
the Bullidse and Phili- 
nidse of Gray.) 
O" 
The aggregate character of the dentition here tabularized is pavimental ; but in Bnllc 
ampulla it is more strap-like, on account of the great breadth of the rachidian plates, and 
the remarkable difference between these and the laterals. 
It would be difficult also to form a judgment of the dentition of Bullcea taken by itself, 
the rachis being altogether absent, and the laterals much reduced in number. 
In the Xudibranchiata, moreover, the pavimental character prevails, although in the 
Ely si Ida? and Eolidw the teeth are reduced to a single strap-like series ; and, considerm 
the collateral relationships of both families, I think that this view is more natural than 
to assume that their dentition is essentially constructed on the type of the strap. "With 
this explanation, the consistency of the present system of determining affinities is quite 
unaffected by the retaining of Eolis and its allies in that position, which their general 
anatomy indicates to be the natural one. On comparing the median row of teeth in the 
broad pavement of Pkyllirrhoe with the single series of Glaucus or Eolis, a close observer 
cannot fail to be struck with their great similarity. According to my own experience, the 
formula of the dentition of Bhyllirrhoe (3.0.3), as given by Mr. Woodward, is incorrect. 
I have, I trust, already sufficiently proved the impropriety of founding a classification 
on the physiology of respiration alone, to say nothing of the peculiar anatomical condi- 
tions respecting it, which we may presume are always present, though not sufficiently 
known to be available. We know, from the analogy of the Crustacea in particular, that 
whether an animal breathe in water (fresh or salt) or in air, the general scheme of its 
structure exhibits no necessary restriction to this habit. The principle here indicated 
(Loven) and Cylindrella (Swn.) 
two 
formula of Bullina as 6*6. 
