208 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



are dovetailed into one another. This is, however, not the case with 

 the external unci of Helicids. In estimating the length of the basal 

 plates this fact must be taken into consideration. 



The basal membrane itself exhibits a reticulum, to be carefully- 

 distinguished from the outlines of the basal plates of the unci. This 

 reticulum may be demonstrated by treatment with silver nitrate, or 

 by staining with Kernschwarz. 



In order to see the details of laciniae and basal plates, and at the 

 same time to avoid the production of an excessive number of false 

 images, it is necessary to mount the radula in styrax. 



Each radula (with few exceptions) presents us with unci of 

 three different types, called central, admedian, and lateral ; central, 

 admedian, and marginal ; or central, lateral, and marginal. (A few 

 other variations of usage may be found.) The terra transitional is 

 also used to indicate unci intermediate in form between the second 

 and third of these. To avoid confusion I shall call those unci of the 

 second type, which lie nearest to the centrals, admedians ; those of the 

 third type I shall call externals. It will not be possible to mistake 

 the application of these terms. When I wish to speak of admedian 

 and external unci together I shall call them pleurce, or pleural elements, 

 the individual unci being pleurals. After careful examination I find 

 I cannot recognize a special transitional type in the pleural unci. It is 

 sometimes possible, in species like H. lapicida, to produce by staining the 

 appearance of three vertical rows of unci in each pleura, but control 

 experiments show that the number of vertical rows taking each 

 combination of colouring matter depends upon the time of staining 

 and the concentration of the stain used. The word transitional may, 

 however, be still useful as an adjective to describe pleural unci 

 intermediate between admedians and externals. 



ADDITIONAL CHAEACTERS. 



[The first description in each case is that of the centrals, the 

 second of the admedians, and the third of the externals.] 



(a) Shape of basal plate. 



Pisana. Wide, oblong; sides very little curved; rounded, upright, 

 oblong. 



hortensis. Long, narrow ; long, narrow ; upright, sub-quadrate. 



nemoralis. Oblongate triangular, all sides curved ; like wing of 

 Sesperia ; sub-tetragonal. 



pomatia. Not regularly incurved except at sides, general shape of 

 a square fitted to a triangle, both with rounded corners ; sub- 

 oblong, not markedly alate ; very much like admedian, only 

 slightly more quadrate. 



aspersa. Truncated triangle, with sinuous sides ; foot-shaped, heel 

 displaced to side of hallux; oblong, nearly square. 



arhustorum. Pointed oblong ; bow-shaped ; irregular tetragon. 



lapicida. Double heart-shape ; rounded truncated triangle ; striated 

 part a perfect rectangle. 



