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



IX. — Remarks on the Dentition of British Pulmonifera*. 

 By Mr. William Thomson, King's College, London. 



[With a Plate.] 



In venturing to offer a few remarks upon the Dentition of the 

 Puliuonobrauchiate Mollusca, 1 do so with much ditfidence, 

 partly on account of the paucity of species to be met with in the 

 British Islands, and the absence of those connecting links with- 

 out which no satisfactory conclusions can confidently be arrived 

 at ; but mainly from the conviction that those who first make 

 observations upon a subject, which had previously been almost, 

 or altogether, neglected, are much more liable to the commis- 

 sion of errors, alike in their microscopical examinations and in 

 their physiological deductions, than those who have a foundation 

 to work upon, be the works of their predecessors ever so erro- 

 neous. It is more, therefore, with the desire of calling attention 

 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 dui'ability of which must 

 be tested by future uialacologists. 



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 px'oposing 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 Academyf.' His observations 

 are however chiefly upon the Marine Gasteropoda. 



HeiT 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 different 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 rolled 

 into a tube (PI. IV. fig. 3 c) ; the posterior end of this tube ia 

 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 



* Read at the Meeting of the British Association in August 1850. 

 t Ofversigt af Kongl. Vetenskaps-Akademiens Forhandlingar, June 

 1847. 



