88 Mr. W. Thomson on the Dentition of British Pulmoiiifera. 



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 (PI. IV. fig. 3), it will be evident that on viewing 

 the whole vertically (PI. 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 plate is of a sub- 

 quadrangular form, often somewhat longer than broad, having 

 its sides slightly hollowed out and its ends nearly straight 

 {Limax) (PI. IV. fig. 5 a), or with its anterior end (that nearest 

 the base of the tubercle) somewhat bow-shaped, in which cases 

 this part overlaps the posterior straight edge of the plate in 

 front of it {Zonites radiatulus) (fig. la). In some it is nearly 

 square {Zonites), while in others it presents the form of an in- 

 verted tapering triangle with a rounded apex {Amphipeplea). 



Tlie form of the tubercle on the central plate is subject to 

 much greater variation than its plate. Sometimes the tubercle 

 is very large and attached to nearly the whole surface of the 

 plate, leaving but a small free apex {Limax) (PI. IV. fig. 5 a) : 

 in other species the tubercle is small and attached by its base 

 to the anterior portion of the plate {Zua) (fig. 8 a) . In another 

 genus {Planorbis) we find that the tubercle is small and has two 

 apices (fig. 9 a). The apex in some few instances projects beyond 

 the edge of the plate, and consequently lies above the base of 

 the tooth next behind it ; but in the majority of cases, the apex 

 of the central tubercle does not project over the edge of its 

 plate. 



The lateral plates not only differ from the central one in form, 

 but also from each other as they approach the edge of the mem- 

 brane. The general form is subquadrilateral, the anterior and 

 posterior edges being subject to the same variations as those 

 described with reference to the central plate, while the inner 

 edge is always more or less convex and the outer edge concave. 

 In those species where the curve of the horizontal row is con- 

 siderable, the plates as they approach the edge get narrower, and 

 in these it is not unusual for them also to assume somewhat an 

 S-form on the one side and its reverse on the other. In others, 

 however, the lateral plates become gradually broader, and 

 eventually twice as broad as the primary lateral plates. 



I come now to speak of the lateral tubercles ; but as they vary 



