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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    to the subject, than with the intention of entering minutely into the form, structure and composition of these teeth, that I am induced to make some brief and general remarks upon them;— as foundation-stones, the friability or durability of which must be tested by future malacologists. I am not aware of any papers having been published in England upon a detailed examination of the teeth of Mollusca, and but very few have appeared upon the continent. Prof. Loven of Stockholm has the credit of first proposing to employ this portion of their oeconomy as a basis of classification, and his excellent paper on the subject may be found in the ‘ Pro¬ ceedings of the Royal Swedish Academy*.^ His observations are however chiefly upon the Marine Gasteropoda. Herr Troschel has published some valuable remarks upon the dentition of some species amongst the Pulmonobranchiata; but (with the exception of some brief notices of the forms of a few unconnected species by difi*erent authors) I know of no other papers of importance in connection with this subject. The tongue of the Pulmonobranchiata generally is a thin ex¬ pansible membrane, two-thirds or three-fourths of which is robed into a tube (PI. IV. fig. 2 c); the posterior end of this tube is closed, while at its anterior extremity the remaining portion of the membrane is expanded into a flattened or spoon-shaped form, which plays against the edge of the horny upper jaw (fig. 2 a), thus acting more in the capacity of an under jaw than a true tongue. It is enclosed in the muscular head of the animal, and is connected with the oesophagus (fig. 2 b) at the anterior end of the tube, the extended upper portion of the oesophagus forming the roof of the mouth, while the expanded surface of the tongue covers the lower part of the mouth. The head is usually globular or nearly so, sometimes slightly attenuated backwards. Prom the junction of the tubes of the oesophagus and tongue, the former passes backwards through the head and leaves it at its upper part behind (sometimes coming out almost at the top of the head), while the tongue takes at once a downward and back¬ ward direction, and protrudes its closed end distinctly at the lower part of the head. If the tubular part of the tongue be laid open and expanded (when it always proves of the same width as the naturally expanded portion), it will be found to be covered on its upper surface with a vast number of plates, each carrying one or more tubercles, which do not stand perpendicularly to the surface of the plates, but are abruptly curved posteriorly, so that the apices * Ofversigt af Kongl. Vetenskaps-Akademieus Forhandlingar, June 1847.