92 Mv. W. Thomson on the Dentition of Britinh Pulmouifera. 



truly Helicine in their conformation. So also are the teeth of 

 all the other Helicidee that I have examined, though they of 

 course present specific characters more or less conspicuous. I 

 imagine however that it will be more difficult to fix upon good 

 generic characters in the teeth of the HelicidcB, than any other 

 family. Zua and Achatina should perhaps come at the end 

 of the list, as their very small central tubercle corresponds with 

 that in the genus Limnceus. The genera Pupa and Vertigo 

 present no apparent difference, and have their central tubercle 

 much of the same form as Zua and Achatina, but in these it is 

 as large as the primary lateral tubercles. 



The character of Limnceus appears to be, to have one small 

 central tubercle, as it were " squeezed up " between two very 

 large lateral ones, each primary lateral having a very large apex 

 internally with a small external one, while at the edge they have 

 altered to one thick prolonged apex projecting inwards and 

 irregularly lobed on its upper edge. Much the same arrange- 

 ment prevails in Amphipeplea, where however the tubercle of the 

 lateral teeth is even still larger, in proportion to its plate. 



Ancylus and Velletia present widely distinct characters, clearly 

 showing that they do not belong to one genus. In Ancylus 

 there are thirty similar lateral teeth in a straight line on each 

 side of the central tooth, and then there is a slight curve through 

 a series of six more teeth where a trifling change in their form 

 occurs. In Velletia, on the contrary, no part of the horizontal 

 row is straight; its central part is much arched, and is com- 

 posed of the central tooth and twelve lateral teeth on each side 

 which do not alter much in form. Then comes one tooth of a 

 different form, and lastly six more on each side, which latter are 

 in a slight curve. 



Physa, again, exhibits a multitude of teeth of a similar form, 

 though different to any that I have seen in other genera ; but 

 unfortunately, owing to the delicacy of the tongue-membrane, 

 I have failed in ascertaining either the form of the central tooth, 

 or the curve of the horizontal row. 



Planorbis appears to be governed (as botanists would say) by 

 the number three. Its primary lateral tubercles have three 

 apices, and the central tubercle, generally in the genus, has two 

 apices placed far apart from each other (PI. IV. fig. 9 a) : this 

 appears to be merely the result of the suppression of the third 

 intermediate apex, a view in which I am borne out by a speci- 

 men of P. marginatus, in which there is only one side apex to 

 the central tooth, the central apex and that on the other side 

 being both suppressed. 



Of Segmentina and a few others I will not now speak, having 

 failed in meeting with glasses good enough to bring out their 



