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 Arcacece, and its close affinity with Nucv.la and Area. The 

 degree of union of the mantle-lobes, and the development of siphonal 

 tubes in this family, as among the neighbouring MytilidcCy is of ge- 

 neric 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 circon- 

 ference inferieure. II est frange sur ses bords, avec de petites taches 

 ou lunules blanches qui alternent avec des stries ray ounces. On voit, 

 au sommet de ce manteau, les impressions denticule'es de la charniere, 

 et en avant et en arriere, les muscles qui unissent les valves. Le pied 

 est grand, robuste, securiforme, tres recourbe en arriere, tranchant et 

 denticule sur son arete, de chaque cote de laquelle sont des laciniures, 

 au tiers anterieur seulement. II ne nous a pas paru se dilater comme 

 dans les muscles. Les branchies sont grandes, libres, subtriangulaires, 

 en pointe, reposant, de chaque cote de la racine du pied, leur doubles 

 lamelles. Les palpes buccaux sont excessivement pctits, reunis dans 

 une partie de leur etendue. L'anus est a I'extremite' d'un court 

 pedicule. La disposition du manteau et le manque de tubes rap- 

 prochent ce mollusque de celui des Nucules, dont il differe cependant 

 par la disposition des branchies et la brievete 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. 



* Vol. iii. p. 476, MoUusques, pi. 78. f. 5. 



