ASHFORD : ON THE DARTS OF BRITISH HELICID^. 75 



salient and their edges still sharp {H. aspersa), then the margins 

 of the blades appear tumid and rounded off {H. pomatia), and 

 finally the thickened edges are cleft, and, opening out, form a 

 double-flanged, angular channel {H. hortensis, fig. 9). In the 

 absence of genealogical affinity it would be difficult, as it seems 

 to me, to account for this connected sequence, especially when 

 we know that foreign forms supply connecting links. Among 

 the British species there occurs no instance of a three-bladed 

 dart. 



One cannot view the exquisitely finished weapon without 

 admiration. We see in its tubular shaft — as also in a stem of 

 grass — the required strength ensured with suitable economy of 

 material, for it is well known that a hollow cylinder is stronger 

 than a solid one, when the areas of the material in the transverse 

 sections are equal. We see, too, a vast accession of strength 

 accruing from the disposition of the blades : an arrangement 

 now adopted in the flanged iron girders of our buildings, though 

 old as the Iris leaf Even the curvature of the shaft gives 

 additional firmness in one direction. These provisions, seconded 

 by the tough thick-walled sheath, ensure the brittle structure 

 from injury till it is required for use. 



Growth. As has already been stated, the dart appears to 

 be formed very rapidly. Keferstein and Ehlers assign, as a 

 ground for this supposition, the fact that immature darts are 

 seldom met with. Semper says that, notwithstanding his 

 numerous dissections, he was never so fortunate as to meet with 

 a weapon in course of formation. Again, I never met with a 

 dart, mature or immature, associated with a manifestly incom- 

 plete shell. Now when we consider that in our climate, and 

 that of central Europe, the shell generally reaches perfection 

 early in the spring, and that coupling also commences early in 

 the year, it will be seen that little interval is left for the forma- 

 tion of the weapon. Perez is decisive and asserts that all is 

 done in about six days. His reasons, deduced from observation, 

 will be found in his pamphlet — " Sur la Generation des Moll. 



