240 ASHFORD : ON THE DARTS OF BRITISH HELICID^. 



instead of channelled, and in showing less regularity of con- 

 vergence from base to point. In some instances the breadth is 

 nearly as great at a point two thirds from the base as at the base 

 itself, and then the intervening boundary lines run nearly parallel 

 or in exceptional cases even concave on both sides. The inferior 

 termination of the blades is quite abrupt, often angled acutely. 

 Amount of curvature and ratio of breadth to length are variable. 

 Schmidt remarks that the two blades in the plane of curvature 

 are broader and sharper in the neighbourhood of the point than 

 the other pair. 



Moquin-Tandon, in his figure of the cruciform section, 

 represents the planes of the two pair of blades as obliquely in- 

 clined to each other. Though I have broken several darts 

 with the special object of observing this feature, no evident 

 instance of the kind has occurred, and we must therefore con- 

 clude the rectangular or nearly rectangular intersection is most 

 common in this country. There is a good engraving on a large 

 scale of part of this dart in Lister's ' Exercitatio Anatomica,' 

 and excellent Uthographs will be found in the plate * accom- 

 panying some notes on the subject in the * Malak. Blatt.' for 

 1850. 



Immature darts occur rather frequently. Three forms in 

 as many stages of growth are shown in fig 7 a, l>, c, and the base 

 of one still further advanced in fig 8. These may with advant- 

 age be compared with corresponding stages in If. aspersa repre- 

 sented on a previous plate. Shaft, blades, blade-margins, base, 

 and annulus appear to follow the usual sequence and it is un- 

 necessary to refer to them more in detail. 



Cases are on record of lost darts entombed in the viscera 

 of this species. Lister remarks ; " Semel ineunte Septembri, 

 duos stimulos unius Cochlese corpore diu, ut conjicio, quod a 

 venere jam diu destiterant, infixos extraxi." On one occasion 

 in the month of May I found in the sac only the lower half of 



* In the British Museum copy this plate is bound up with the volume 

 for 1852. 



J.C.iv., Oct., 1884, 



