112 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



Family 11. TRIGONACEA. 



Testa sequivalvis ; cardine dentibus paucis lamelliformibus, valde inter- 

 sertis. 



Lamarck's family of "Les Trigonies" includes two genera, Trigonia and 

 Castalia ; the former is sufficiently distinct to claim its arrangement as a 

 separate family, but the latter evidently belongs to that of the Naiades ; 

 they differ also in their natural places of habitation, one being marine, 

 the other fluviatile. At the time that these two genera were united in 

 one family by Lamarck, the shell of each was exceedingly rare, and 

 their animals unknown. Abundant opportunities have, however, been 

 since afforded for examining them ; and the result shows that the Tri- 

 gonia are not only eminently distinct from the Castalia, but that the 

 latter, belonging to the family of the Naiades, are, in fact, true spe- 

 cies of the genus Unio ; Castalia of Lamarck therefore must be aban- 

 doned. 



The Trigonacea may be described as having an equivalve shell, with 

 the hinge composed of a few broad lamelliform teeth, closely interlocking 

 with each other ; the following genus is the only one that can be referred 

 to this family : 



Trigonia. 



TRIGONIA, Bruguiere. 



Testa aequivalvis, insequilateralis, trigona, intiis iridescenti-margaritacea, 

 extiis costis aut sulcis, vel transversim vel longitudinaliter dispositis, 

 obtecta. Cardo dentibus oblongis, lamelliformibus, divaricatis ; in 

 valva altera duobus, utroque latere transversim sulcatis ; in altera 



