Description of Trigonia.
The mantle-lobes are rounded and plaited, to correspond with the ribs of the shell. The edges of the mantle are marked with white spots; posteriorly, opposite the anus they are provided with short convex appendages. The mantle-lobes are disunited throughout, not joining until they reach the upper surface of the posterior adductor, some distance above the anus.
The gills are somewhat triangular, extending backwards almost horizontally on each side of the visceral mass. Each gill is formed of three stems, fixed at one extremity, free and pointed at the other, and giving attachment throughout their whole length, on one side to depending filaments, which become shorter as they are more posterior. The filaments are formed of a tubular horny thread, supporting on one side a broad membranous fringe. I could perceive no trace of vessels in this fringe, but it appeared to be covered by an epithelium (ciliated ?).
The mouth is placed at the anterior and superior part of the animal, between two thickish horizontal lips. The labial tentacles are two on each side, rather long, lanceolate, and slightly pectinated. The anus is placed posteriorly and superiorly between the gills, and just above the posterior adductor muscle.
The so-called "foot" is composed of two portions, an upper and [8] quadrilateral (properly the abdomen), and a lower pointed part (the true foot), the two being set at right angles to one another.
The first portion is sharp-edged and slightly pectinated posteriorly, marke by a groove bounded by two folded lips anteriorly. The second portion is slightly pecitnated along its lower edge, pointed anteriorly, prolonged behind into a curved process, where it joins the superior portion.
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