12 



The siphons differ slight]} 7 in structure ; the dorsal, or 

 exhalent one (Si.d., figs. 2 and 3), is the shorter of the two. 

 Its free edge is quite even and a small portion of the 

 wall of the tip is exceedingly thin. This thin tip is 

 contractile, and is generally closed forming a little cone at 

 the end of the siphon. The tentacles, which are rather 

 over 1 mm. in length when fully extended, are situated in 

 an irregular ring at the base of this cone. Other and 

 larger tentacles are borne on the wall behind this ring. 

 At intervals the conical tip of the siphon opens and water 

 and faecal matter are suddenly expelled. The ventral or 

 inhalent siphon [Si.v., figs. 2 and 3) remains permanently 

 open. The free edge bears a great number of very delicate 

 tentacles, smaller and thinner than those borne on the 

 outer wall and differing slightly in structure. Behind 

 this, as in the upper siphon, is a ring of tentacles with 

 others scattered irregularly upon the wall. At the tip of 

 both upper and lower siphons are a number of brown pig- 

 ment spots which are the openings of little pigmented 

 crypts or glands. 



The mantle edge (fig. 23) is thrown into longitudinal 

 folds extending along its entire length. There is an inner 

 strong fold projecting into the mantle cavity, a smaller 

 median fold, and an outer fold which dips into the 

 grooves on the shell margin. On the surface applied 

 to the shell, the epidermis consists of rather irregular 

 cubical cells, except towards the extreme edge, where the 

 cells become spindle shaped and are crowded with brownish 

 pigment granules. Over the rest of the mantle edge 

 (ventral and internal surfaces), there is an epidermis 

 consisting of a very regular layer of cubical cells with very 

 distinct nuclei and a strong cuticle. On the inner of the 

 three folds mentioned above, and in the ' deep groove 

 separating this from the small median fold, the cuticle 



