FAMILY 11. TRIGONACEA. 113 



quatuor, uno latere tantum sulcato. Ligamentum externum, eras- 

 sum, marginale. 



It was not till some time after the introduction of this genus by Bru- 

 guiere, known to him only in a fossil state, that a recent species was 

 discovered ; and we are again indebted to M. Quoy, on his return from 

 a voyage of discovery in the ship ' Astrolabe,' for the first notice of the 

 animal. His observations on the anatomy of the Trigonia tend to esta- 

 blish a direct affinity between this family and the Arcacea, and singularly 

 confirm the conjectural opinion of Lamarck, who, in speaking of the 

 NucuIcb, says, " et forrnant une transition e'vidente aux Trigonies, elles Kent 

 ces dernieres a lafamille des Arcace'es." 



The shell of Trigonia may be described as being equivalve, inequilateral, 

 and somewhat triangular, having the interior lined with the most lustrous 

 iridescent pearl, and the exterior covered with ribs or furrows, running 

 longitudinally in the only recent species known, but transversely in some 

 of the fossil species. The hinge consists of two oblong, divaricate, lamel- 

 liform teeth in one valve, transversely sulcated on both sides ; and four 

 in the other, sulcated on one side only. The ligament is external, thick, 

 and marginal. 



Examples. 



PL LXXXVI. Fig. 1 and 2. 



Trigonia margaritacea, Lamarck, Annales du Mus., vol. iv. p. 355. 



pi. 67. f. 2. Quoy and Gaimard, Voy. de 1'Astrolabe, Mollusques, 



pi. 78. f. 1 to 4. 

 Trigonia pectinata, Lamarck (Anim. sans vert.). 



PI. LXXXVI. Fig. 3. (fossil.) 



Trigonia costata, Lamarck, Anim. sans vert., new edit., vol. vi. p. 517. 

 Enc. Meth., pi. 238. f. I. a, b, and f. 2. a, b, c. 



VOL. I. Q 



