Remarks on the dentition of British pulmonifera / by William Thomson.

  • William Thomson, 1st Baron Kelvin
Date:
[1851]
Catalogue details

Licence: Public Domain Mark

Credit: Remarks on the dentition of British pulmonifera / by William Thomson. Source: Wellcome Collection.

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    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. I 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 Flanorhis contortus (fig. 12), or divaricate from each other at the central tooth, upwards as in Achatina acicula (fig. 15), or downwards as in Ancylus fluviatilis (fig. 13). Wherever the curve presents great angularity (as in Zonites radiatulus) (fig. 14), there we find a sudden change in the form of the teeth, while in like manner a gradual curve is the result of a gradually pro¬ gressive change in the form of the teeth, the degree of deviation from a straight line being exactly in proportion to the amount of change which takes place between the form of the central and edge-teeth. It may perhaps seem that I have dwelt at greater length on this point than was necessary; but as there are many species of Pulmonobranchiata so small as to render it difficult with the best glasses to determine the form of the plate and often of the tubercle, the attachment of the tubercle to the plate being the only part clearly visible, it appears to me desirable that the fol¬ lowing rules should be laid down with reference to the form of the lateral teeth, in connection with the horizontal rows.
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