142 Zoological Society. 
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 zoolo- 
gical division of the Voyage of the Astrolabe*. Since then I 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 almost as much importance as the descrip- 
tion of an unobserved animal, the Zoological Society may consider 
Mr. Huxley’s notes in the light of a valuable contribution to mala- 
cology. 
Both accounts confirm the idea suggested by the shell of its position 
among the Arcacee, and its close affinity with Nucula and Arca. The 
degree of union of the mantle-lobes, and the development of siphonal 
tubes in this family, as among the neighbouring Mytilide, is of ge- 
neric and not sectional significance. 
I add the description of the animal given by the French naturalists 
for comparison :— 
«T’animal a le manteau ouvert dans les trois quarts de sa circon- 
férence inférieure. I] 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 arriére, les muscles qui unissent les valves. Le pied 
est grand, robuste, sécuriforme, trés recourbé en arriére, tranchant et 
denticulé sur son aréte, de chaque cété 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 coté de la racine du pied, leur doubles 
lamelles. Les palpes buccaux sont excessivement petits, réunis dans 
une partie de leur ¢tendue. L’anus est a lextrémité d’un court 
pédicule. La disposition du manteau et le manque de tubes rap- 
prochent ce mollusque de celui des Nucules, dont il différe 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. Lach 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. 
* Vol. iii. p. 476, Mollusques, pl. 78. f. 5. 
