( 15 ) 



DIVISION IX. TBIGONEiE. 



Ligament 



GENERA. 



I. Tbigonu. Brug. M. 



Equivalved, inequilate- Cardinal teeth oblong, 

 ral, trigonal, sometimes flattened on the sides, marginal. (1) 

 sub-orbicular. Fig. 48. diverging and furrowed 



transversely ; the 2 on 

 the right valve furrowed 

 on each side, and the 

 4 on the other valve 

 furrowed on one side 

 only. 



exterior, 



2. Cast alia. Lam. F. 



Equivalved, inequilate- 

 ral, trigonal ; beaks na- 

 ked, curved back. 



Hinge with 2 lamelii- 

 form teeth, transversely 

 striated ; one posterior, 

 distant, shortened, sub- 

 trilamellate ; the other, 

 anterior, elongated, la- 

 teral. 



Ligament exterior. (2) 



(1) The only living species w as found by Peron at King's Island, New-Holland. 

 It is covered with mother of pearl within ; is a shell of singular beauty, and valued 

 at a high price. Foss. in Crag Marl, Green Sand, Upper Oolite, Under Oolite; at 

 Puy-de-Domej Mans, Coulainesj Courtagnon, Gundershofen, Havre t Caen. 



(2) This shell seems to form a link between the Trigonia and Naiades; it is im- 

 perfectly known from there being but one of the kind, in the Cabinet of M. le 

 Marquis de Dree, which is not at present accessible. Habitat unknown. 



