74 ASHFORD : ON THE DARTS OF BRITISH HELICID^. 



In some species the annulus is permanently absent. The dart 

 consists of a straight or curved, sometimes slightly twisted, 

 tubular shaft of carbonate of lime, tapering to a fine, solid, 

 transparent point above (fig. 8), and enlarging gradually, or 

 more often somewhat abruptly to the base (fig. 7), where it 

 assumes the form of a sub-conical cup whose roughly indented 

 margin rests upon and fits the superior margin of the annulus 

 (fig. 6). The apex of the tubercle thus projects into the 

 expanded base of the dart, attachment between the two being 

 further secured by an intervening viscid secretion, which also 

 extends in the form of a glutinous thread some little way up 

 the central perforation of the shaft. The sides of the shaft are 

 sometimes furnished with blades symmetrically and longitudi- 

 nally disposed; they serve to brace the stem and are not 

 intended for cutting. A quarter-inch objective clearly reveals, 

 under favourable light, the crystalline character of the edges of 

 incomplete blades, but is insufficient for detail. The reader 

 who requires a strictly technical description of the dart and its 

 sheath is referred to Prof. C. Sempers's " Beitrage zur Anatomie 

 und Physiologic der Puhnonaten," Leipzig, 1856. 



Form. Between the almost linear dart of H. caperata^ 

 devoid alike of blades and conical base, and the complicated 

 weapon of H. Pisana with its four channel-edged blades, is a 

 series of passage-forms, constant in each species, but connected 

 in a way that poinls definitely to a course of progressive 

 development. The first step is indicated by a slight compression 

 of the region near the point (Z^ ertcetorum), the next by a 

 further expansion into a lanceolate head with blunt edges 

 {H. arbustorimi). A thinner head with sharp edges {H. lapicida) 

 conducts to a form with two slender blades springing from a 

 circular shaft i^H. virgata). The next stage is readied by an 

 enlargement of the pair of blades both as to prominence and 

 length, and the addition of another but less prominent pair, 

 disposed in a plane at right angles to that of the primary 

 couple {H. nonoralis). We next find the four blades equally 



J.C, Iv., July, 1883. 



