86 CONCHIFERA. DIMYAIRA. 



FAMILY III. TRIDACNITES. 



Shell transverse, equivalve, with the muscular impression under 

 the middle of the upper margin, and prolonged on each side. 



The shell of this family is regular, equivalve, solid, and always distinguishable 

 from its sinuous or waved margins. The largest known shell belongs to this group. 



Gen. 30. HIPPOPUS, Lam. Chama^ Lin. 

 Shell equivalve, regular, inequilateral, transverse ; hinge with 

 two compressed, unequal teeth ; ligament marginal, exterior ; 

 no open space at the hinge. 



H. maculatus, Lam. (C. hippopus, Lin.) Shell transversely ovate, 

 longitudinally plaited, ribbed, and muricated; posterior si ope heart- 

 shaped, closed, and the margin toothed. 2 to 1 2 inches long. In- 

 habits Indian seas. -D'Argenv. pi. 23, fig. H. 



Gen. 31. TRIDACNA, Lam. Chama, Lin. 

 Shell regular, equivalve, inequilateral, transverse ; hinge with 

 two compressed teeth; ligament marginal, exterior; a round- 

 ed dentated open space at the hinge. 



The opening in the posterior marjrin of the shell serves the animal for the projec- 

 tion of the byssus, by which it attaches itself to rocks. There is but one transverse 

 muscular impression on the shell. Fossil species of this genus sometimes occur. 



T. gigas, Lam. Shell large, transversely ovate, with remote broad, 

 rounded longitudinal ribs, armed with vaulted scales, and the 

 posterior slope heart-shaped and gaping. Inhabits Indian Ocean. 

 Rumph. Mus. pi. 43, fig. B. 



This shell, the largest and heaviest known, varies much in size and weight. Oiie 

 described by Linnaeus weighed 498 pounds ; and another from Sumatra preserved 

 at Arno's Vale, Ireland, according to Mr Dillwyn, weighed 507 pounds weight. The 

 largest valve of this specimen measured 4 feet 6 inches in length, and 2 feet 5^ in- 

 ches in breadth. In the church of St Sulpice at Paris is a specimen used as a lont, 

 which was presented to Francis I. by the Venetians. Large pearls are occasionally 

 found in this species ; and one exhibited at the late Sir Joseph Banks's in June 1814 

 was valued at from L. 200 to L. 300. 



T. elongaia, Lam. Shell ovate-oblong, produced behind ; with ribs 

 of imbricated scales ; scales thick, semi-elevated ; posterior aper- 

 ture large ; colour white or yellowish, with the interstices more 

 or less longitudinally striated. 6 inches long. Inhabits Indian 

 Ocean. Lam. vi. 1, 106. 



T. squamosa, Lam. Shell ovate, white, the young shell reddish, 

 with scaly ribs ; the scales large, erect, distant, and the inter- 

 stices thickly striated; opening small, and the internal border cre- 

 nated. Inhabits Indian Ocean. Rumph. Mus. pi. 43, fig. A. 



ORDER II. DIMYAIRA. 



Shell with two separate and lateral muscular impressions. 



This order, including the greater portion of the Conchifera, comprehends those 

 itaceous animals attached to the shell by at least two muscles, widely separated, 



