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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    of these projections invariably point towards the closed end of the tongue (PL IV. figs. 3 & 4). These teeth are distributed in rows all over the membrane^ and are closely packed together. The longitudinal rows always consist of straight lines, but the transverse rows are variously curved, often boW'Shaped, sometimes angular, rarely straight (figs. 12--19). The degree of curvature of the transverse row, and the varia¬ tions which the curves show (being sometimes composed of arcs of circles, while at other times they are made up of short straight lines lying in different directions), appear to depend on the form of the teeth. I shall allude to this again, after having described the teeth. The number of teeth in a row does not seem to be always the same in individuals of the same species, though it may be pro¬ nounced as constant within certain limits. In different species, however, it is exceedingly variable: as a rule, there are more teeth in a longitudinal than in a transverse row, usually one- third or one-fourth more, though in Helix Pomatia the number in the transverse row exceeds that of the other, while in Lim~ nceus stagnalis there are 110 in each direction. Of the rows, taken longitudinally, I need not say much, it being more easy to explain the variations in the teeth, when the rows are regarded transversely. Suffice it to mention, that in the centre of the membrane there is a longitudinal row of teeth of different form to any of the rest. It is to the form of the central tooth of the transverse row (the series of which constitutes the central longitudinal row just referred to) that I would wish to draw more particular attention; as I hope to show presently that all the other teeth partake more or less of the form of this tooth. But before proceeding to its description, it will be well to explain the precise meaning of the terms I purpose using. From reference to an ideal vertical longitudinal section of a plate with its tubercle (PL IV. fig. 3), it will be evident that on viewing the whole vertically (PL iV. fig. 4) through the microscope (the object being almost transparent), three outlines will generally be seen, that of the plate, that of the attachment of the tubercle to the plate, which I shall refer to as the base, and that of the free point of the tubercle, which I shall speak of as the apex: the tooth will therefore be regarded as the plate and tubercle com¬ bined. The central plate and its tubercle differ from all the others on the membrane in being symmetrical. The piate is of a sub- quadrangular form, often somewhat longer than broad, having its sides slightly hollowed out and its ends nearly straight A 2
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