Mr. W. Thomson on the Dentition of Bi-itish Pulmonifeva. 87 



the roof of the mouth, while the expanded surface of the tougue 

 covers the lower part of the mouth. The head is usually globular 

 or nearly so, sometimes slightly attenuated backwards. From 

 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 

 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 dififerent 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 moi*e, though in Helix Pomatia the number 

 in the transverse row exceeds that of the other, while in Lim- 

 neetis 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. 



