Mr. W. Thomson on the Dentition of British Pulmonifera. 89 



nearly as much in the same individual as they do in different 

 species, it would be an almost endless task to describe all the 

 forms they assume. A careful examination of them generally 

 shows, I think, clearly, that the following rule may be laid down 

 regarding their form. 



If an ideal line be drawn longitudinally through the central 

 tubercle, so as to divide it equally, it will be found that the two 

 halves are precisely similar (PI. IV. fig. 4) ; but such is not the 

 case with any of the lateral tubercles. We find, too, that those 

 lateral tubercles which are nearest to the central tubercle are 

 always more similar to it in general form than those at the 

 edge ; indeed, that the tubercles become more unlike the central 

 tubercle as their position is nearer to the edge of the membrane. 

 Hence I deduce the following rule : viz. that the lateral tubercles 

 are merely modifications of the form of the central tubercle ; and 

 that these modifications are effected by the suppression of the 

 prominences on the inner side of each lateral tubercle, and the 

 simultaneous increase of the corresponding parts on the outer 

 side. By the "inner" and "outer" sides, I mean the side 

 nearest to, or farthest from, the central tooth. 



In Limax carinatus and some allied species we have this rule 

 clearly exemplified, the change from the typical form into that 

 at the edge being very gradual, and showing every possible 

 connecting link (PI. IV. figs. 5, 6, 7). But this gradual pro- 

 gression is far from being the case in all species : the sudden and 

 abrupt change in form which is seen in the fourth lateral tuber- 

 cle of Zonites radiatulus (fig. 1 c) might at first sight seem to 

 overthrow this rule ; but on comparing this tongue with others, 

 where the central tooth is somewhat similar and the modification 

 of the lateral tubercles more gradual, it will be at once perceived 

 that this sudden change of form is owing to the absence of the 

 connecting links, which a reference to the progressive alteration 

 in other species will readily supply, if not actually, at any rate 

 to the imagination ; and it will be found that an application of 

 the rule I have laid down, to the third lateral of Zonites radia- 

 tulus, would eventually bring out the form of the fourth lateral, 

 though the connection would, I grant, require several plates to 

 complete it. I would now be permitted again to refer to the 

 directions assumed by the horizontal rows, which (as I previously 

 mentioned) depend upon the form of the teeth. Wherever a 

 straight line is observable in the arrangement of the lateral teeth, 

 it will be found that all the teeth in that line are similarly 

 formed, whether the right and left laterals are in the same line 

 as in Planorbis contortus (fig. 12), or divaricate from each other 

 at the central tooth, upwards as in Achatina aciada (fig. 15), or 

 downwards as in Ancijlus fluviatilis (fig. 13) . Wherever the 



