74 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 Ojnsthobrcmchiata that 

 any difiiculty in this respect presents itself. Can anything, for example, be more diver- 

 sified than the characters of the lingual dentition in the BnllidcB of authors ? No less than 

 four distinct types are distinguishable in this family, thus : — 



Central series absent. Central series present. 



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 



BuLL.'EA. Aplustrum. Cylindrella. Amphisphyra. 



(Philinidse of Gray.) (Buhinidre and Amplustridas (Bullidse of Gray.) (Occurring respectively in 



of Gray.) the BuUidce and Phih- 



nidse of Gray.) 



The aggregate character of the dentition here tabularized is pavimental ; but in Bulla 

 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 Bulloea taken by itself, 

 the rachis being altogether absent, and the laterals much reduced in number. 



In the Nudibranchiata, moreover, the pavimental character prevails, althotxgh in the 

 ElysiidcB and BoUcIcb the teeth are reduced to a single strap-like series ; and, considering 

 the collateral relationships of both famiHes, 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 aflinities is quite 

 \inaffected by the retaining of ^olis 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 oi PhylUrrhoe with the single series of Glancus ov Bolls, a close observer 

 cannot fail to be struck with their great similarity. According to my own experience, the 

 formula of the dentition of PhylUrrhoe (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 tlie peculiar anatomical condi- 

 tions respecting it, which we may presume are always present, though not sufiicieutiy 

 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 restrictioii to this liabit. The principle here indicated 



* Cylichna (Lovcn) and Cyliyidrella (Swn.) are regarded as synonyms of Bullina (Risso), though the two former 

 genera present a central series in the lingual pavement, while Dr. Gray gives the dental formula of Bullina as 6'(i. 



