EXTERNAL CHARACTERS. 309 



the centre of the dorsal surface of the mantle the shell is visible through the 

 large triangular shell-foramen. Tliis opening, (Plate 6, fig. 1, I) is nearly of 

 the form of an isosceles triangle, the apex directed forward, and measures 23.5 

 mm. in length. In Plate 6, fig. 1, the relations of the mantle to the other organs 

 of the pallial cavity are represented, the ctenidium, however, being omitted. 

 The epipodia have been removed, the outline of their bases being represented 

 by the continuous line a, while the dotted line k indicates the projection of their 

 upper margins upon the structures below. The mantle is attached to the dor- 

 sal wall of the body to the left of the dotted line c, which represents the 

 outUne of the shell, the triangle I indicating the widely open foramen of the 

 latter. Near the median line in front, above and to the left of the spermatic 

 furrow i, the free margin of the mantle begins, and is shown by the continuous 

 line d, which passes in a gentle curve to the right and backward, then more 

 sharply to the left side, where it is continued into an erect auriculate lobe, the 

 siphon, s. The siphon is highest on the left side, and behind rapidly descends 

 around the anal opening, /, as a thin low fold of integument, which continues to 

 the right and forward as a stronger fold, g, 2.5 imn. in height, along the floor of 

 the branchial cavity to the common .reproductive opening h, the upper margin 

 of which it forms (Plate 4, fig. 3, a). The ventral surface of the mantle is very 

 glandular and of a dark brown color. Toward the posterior margin it is thick- 

 ened and reflected in a prominent flap e, as noted by Gilchrist (1894), for D. 

 rumphii [= scapula]. In the Easter Island species, however, the reflected lobe 

 is present only at the right posterior angle of the mantle, where it is quite large, 

 the reflected margin extending so far as to overlie a portion of the shell-cavity. 



The rear wall of the palUal chamber is formed by the union of the epi- 

 podial lobes behind the siphon, the main portion of the oblique posterior disc 

 of the body being made up of the fused epipodia. The round excurrent foramen 

 is 20 mm. above the floor of the pallial cavity at the base of the posterior wall. 

 Above the mantle the roomy palUal cavity extends downward upon the left 

 side nearly as far as upon the right. 



Figure 5 of Plate 6 represents the palhal organs as disclosed by the removal 

 of the most of the mantle. It has been cut away along the double line m, which 

 represents its section close to its origin from the dorsal body-wall. As in Plate 6, 

 fig. 1, the line of the base of the epipodia is represented by a, the dotted Une k 

 indicates the projection of their free upper margin, while b is the circumference 

 of the oblique posterior disc. Below the mantle the relatively large ctenidium 

 is shown, its free portion curving backward in the palUal cavity in almost a 



