32 



LAND A.ND FRESHWATER 



The ceutre tooth is rounded above, with straight sides sloping 

 outwards to the broad base, tricuspid, but on some the large side 

 cusp is replaced by two smaller ones; the second tooth is very 

 rounded above into three large cusps ; the second tooth has nearly 

 parallel sides, sharply bent over on the cuspid edge, with four sharp 

 narrow cusps ; the third is made up of two very distinct portions, 



■)/^^. 



Fig. 2. — Tropidoplwra hddleoensis. 



the inner one having 6-8 sharp-pointed teeth on a curved edge ; 

 then intervenes a long slit, and the next and outermost part is a 

 curved edge, set with about twelve very fine saw-like cusps. Jn every 

 respect it is like unifasciata, except that the outermost tooth is not 

 so decidedly divided into two, the formula being 



1 2 3.3' 



4 



±^1 

 12- 



?— 12 



Fig. 3. — Cyclostoma elegans. 



In connection with this detail of structure, a comparison of the 

 radulaof our English species, C. elegans (fig. 3), the type of Cyclostoma, 

 is interesting as regards this almost complete and partial division of 



