THE accompanying account of the animal of Trigonia was forwarded to me by Mr. Huxley, Assistant-Surgeon to the Rattlesnake, now surveying in the Eastern and Australian Seas, under the able command and scientific zeal of Captain Owen Stanley.
The great number, beauty and geological importance of the species of this interesting genus have made especially valuable a knowledge of the structure of its animal. Quoy and Gaimard were the first to give any account of it, and a figure and description of the animal of Trigonia were published from their drawings and notes in the zoological division of the Voyage of the Astrolabe.2 Since then l am not aware of this curious creature having been re-observed, though much has been written respecting its systematic position. As in such a case a verification of the evidence we possess, through a new and accurate set of observations, is of alinost as much importance as the description of an unobserved animal, the Zoological Society may consider Mr. Huxley's notes in the light of a valuable contribution to malacology.
Both accounts confirm the idea suggested by the shell of its position among the Arcaceæ, and its close affinity with Nacula and Arca. The degree of union of the mantle-lobes, and the development of siphonal tubes in this family, as among the neighbouring Mytilidæ, is of generic and not sectional significance.
 I add the description of the animal given by the French naturalists for comparison:
"L'animal a le manteau ouvert dans les trois quarts de sa circonférence inferieure. Il est frangé sur ses bords, avec de petites taches ou lunules blanches qui alternent avec des stries rayonnées. On voit, au sommet de ce manteau, les impressions denticulées de la charniere, et en avant et en arrière, les muscles qui unissent les valves. Le pied est grand, robuste, sécuriforme, très recourbé en arrière, tranchant et denticulé sur son aréte, de chaque côté de laquelle sont des laciniures au tiers antérieur seulement. Il ne nous a pas paru se dilater comme dans les muscles. Les branchies sont grandes, libres, subtriangulaires, en pointe, reposant, de chaque côté de la racine du pied, leur doubles lamelles. Les palpes buccaux sont excessivement petits, réunis dans une partie de leur étendue. L'anus est a l'extremite d'un court pédicule. La disposition du manteau et le manque de tubes rapprochent ce mollusque de celui des Nucules, dont il diffère cependant par la disposition des branchies et la briéveté des appendices de la bouche."
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  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.
Visceral mass.The mouth opens by a very short sophagus into a wide pyriform stomach, surrounded by a dark dendritic liver. The stomach narrows into a long intestine, which descends for the whole length of the abdomen, and forms one or two loops in the substance of the generative gland; then passes up again above the stomach, penetrates the heart, and passing between the two small lateral muscles of the foot, terminates in the anus.
1 The paper is by Prof. E. Forbes. The part headed Description of Trigonia is written by Huxley.Eds. 2 Vol. iii. p. 476, Mollusques, pl. 78, f. 5 3 The word Plate accompanied by a number and printed within brackets refers throughout to the plate number assigned to each plate in the present reprint of Huxley's papers, and not to the original numbering of the plates as published in transactions or journals.Eds.
1 The paper is by Prof. E. Forbes. The part headed Description of Trigonia is written by Huxley.Eds.
2 Vol. iii. p. 476, Mollusques, pl. 78, f. 5
3 The word Plate accompanied by a number and printed within brackets refers throughout to the plate number assigned to each plate in the present reprint of Huxley's papers, and not to the original numbering of the plates as published in transactions or journals.Eds.
Fig. (1.) View of the animal with the right valve of the shell removed, and the right lobe of the mantle turned back. a, mouth; b, anus; c, filamentious appendages of mantle; d, gill; e, grooved superior part of foot.
Fig. (2.) View of the animal from behind, with the valves separated. Letters as before.
Fig. (3.) Visceral cavity laid open. a, stomach, surrounded by the liver; b, intestine; c, heart; d, generative gland.