72 A'SHFORD : ON THE DARTS OF BRITISH HELICIDiE. 



the inversion of the base of the whole organ towards its aperture, 

 which admits of considerable lateral expansion, the dart is 

 protruded outwards for use. Among our British reprentatives 

 are a few species exhibiting a second pouch interposed between 

 the teliferous sac and the vagina, to both of which it more or 

 less adheres. This accessory appendage is never furnished with 

 a weapon, and forms, with the dart-sac proper, the bi-lobed 

 arrangement already referred to. It is best examined in 

 H. rtifescens. 



Growth. The dart-sac does not make its appearance till 

 the animal has entered upon free life and made some advance 

 in growth. I can find no trace of it in the young H. aspersa of 

 5 or 6 mm. in diameter. When the shell of that species has 

 attained ID mm. the incipient organ presents itself as a small 

 protuberance on the outer side of the vaginal tube. In one of 

 12 mm. I have found it still only 0.4 mm. in length and as 

 broad as long, increasing to 1.25 mm. as the shell progresses to 

 16 mm. (the size of a threepenny piece). At this stage the 

 inner coat is clearly differentiated, and when the shell has grown 

 to 20 mm., the former is a soft roundish body about 0.66 mm. 

 long, with slight parietal attachment, but with a perceptible 

 tubercle at its base. As the outer envelope increases in size and 

 consistency, the inner coat assumes the shape of a sugar-loaf, 

 then becomes pointed at the apex and grows more rapidly than 

 the outer till the two are of nearly equal length. By this time 

 the shell, judged by its completed peristome, will have reached 

 maturity. As the pairing time approaches a rapid change takes 

 place. The whole organ acquires a much firmer muscular 

 condition, and the colouring matter —in such species as possess 

 it — is developed, first as a light pink round the neck of the sac, 

 then as a reddish or bluish-brown more generally diffused, 

 ending in the full-tone. It appears to me to be during the later 

 stage of coloration, or the corresponding period of growth where 

 coloration does not occur, that the dart is formed. The first 

 intimation of the presence of the two dart-sacs of H. riifescens is 



J.C, iv., July, 1883. 



