132 ASHFORD : ON THE DARTS OF BRITISH HELICID^. 



This and the last species have been put in sequence for 

 the sake of contrast. 



The curious broadly ovate dart-sac of H. Pisana has a 

 prominent, medial, external ring suggesting the form of an 

 acorn (fig. 9). From near the junction of the sac with the 

 vagina spring the two immense, subclavate, simple mucous 

 glands. These are of slightly irregular outline, white or tinged 

 with buff, present a tumid, more or less soft spongy appearance, 

 and are about 10 mm. in length. In Schmidt's figure (Stylomm. 

 fig. 23) they are represented very much more slender than I 

 have ever met with them in home specimens, and he describes 

 them as "vvormformig." It would be interesting to know if a 

 local difference of this kind is constant ; but it is possible that 

 the preparation, from which the figure referred to was drawn, 

 — and which the eminent malacologist acknowledges was one 

 that had been some time mounted, — had contracted in drying. 



I have never met with either gland bifid or showing a 

 tendency to cleavage. Moquin Tandon mentions one instance 

 of the kind. The differentiation of the dart-sac appears to be 

 first observable when the shell is between 6 and 8 mm. in 

 breadth. 



The dart of this species is very small for the size of the 

 animal ; it does not exceed in length that of H. caperata. 

 The four symmetrically disposed blades are thin and translucent, 

 presenting, under a quarter-inch objective, the appearance of a 

 multitude of minute crystalline spicules. Their outer edges 

 diverge into two well-formed flanges, inclined to one another at 

 an angle of about i5o°—i6o°, thus forming troughs or 

 channels (figs. 10 and 11). Each compound blade disappears, 

 not very gradually, before reaching the point, but ends quite 

 abruptly and squarely at the base of the dart about half-way 

 down the expanded part (fig. 16). Such is the complicated 

 build of this beautiful little obelisk when perfectly formed. It 

 is however, subject to many modifications of detail, though 

 these are never of such a kind as to obscure the type or plan of 



J.C, iv., Jan., 1884. 



