EXTERNAL CHARACTERS. 309 
the centre of the dorsal surface of the mantle the shell is visible through the 
large triangular shell-foramen. This opening, (Plate 6, fig. 1, 1) 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 
outline of the shell, the triangle / indicating the widely open foramen of the 
latter. Near the median line in front, above and to the left of the spermatic 
furrow 7, 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, f, as a thin low fold of integument, which continues to 
the right and forward as a stronger fold, g, 2.5 mm. 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 pallial 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 pallial cavity extends downward upon the left 
side nearly as far as upon the right. 
Figure 5 of Plate 6 represents the pallial 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 line 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 pallial cavity in almost a 
