TRID ACN A 



Plate I. 



Genus TRIDACNA, Briig}uere. 



Testa ovata vel ohlortyo-ovula, aquivalvis, inaquilateralis, 

 atitice ampliler hians, radiaUm pauci-costala, contis 

 phrumqne aqiiamaih. Cardo clenlibus dvobiis in ntrd- 

 que vulva comprcssis, inaqiialihus, posticis, inserlis. 

 Ligamentum submarginale, externum. 



Shell ovate or oblong-ovate, equivalve, inequilateral, 

 largely gaping anteriorly, radiatcly few-ribbed, ribs 

 generally sealed. Hinge with two posterior com- 

 pressed interlocking teeth in each valve. Ligament 

 submarginal, external. 



The TridacnfF, or Furbelowed Clams, one of which, 

 T. gigas, is the largest of all mollusks, live in beds of some 

 extent in lagunes among coral reefs, among the islands of 

 the Eastern and Pacific seas. The shell is generally white, 

 sometimes tinged with red and saffron or lemon-yellow, 

 but the animal is brilliantly coloured. M. Quoy describes 

 the beautiful iridescent glare of blue, violet, and yellow, 

 variegated with fantastic markings, that is presented by 

 these submarine parteiTes as seen through the clear blue 

 water; and Mr. Cuming speaks with enthusiasm of passing 

 over a mass of them nearly a mile in extent, which resem- 

 bled nothing so much as a beautiful bed of tulips. 



With the exception of T. gigas, from Navigators' Is- 

 lands, all the species of which I liave the habitat, are as- 

 signed to the Moluccas or Philippine Islands. The shells 

 are mostly furbelowed with large high-va\dted scales, and 

 all are white, occasionally tinged with red and safl'ron or 

 lemon-yellow. The lunulc of the shell is widely open for 



the passage of a foot capable of spinning a powerful ten- 

 dinous byssus. 



Species 1. (PI. I. Fig. 1 a ; PI. II. Fig. 1 h, c, Mus. 

 Cuming.) 



Tridacna gigas. Trid. testa maxima, antice snlobliqjie 

 prodnctd, quadricostatd, costis dense sqtiamatis, sqita- 

 mis nmuerosis, creberrimis, eetate brevibns, plus minus 

 obsoletis, interstitiis subobsciiri; radiatim liratis ; alba, 

 intiis inierdvm rosed. 



The giant Tkidacna. Shell very large, rather obliquely 

 produced anteriorly, four-ribbed, ribs densely scaled, 

 scales numerous, close-set, short and more or less 

 obsolete with age, interstices rather obscurely radiatcly 

 ridged ; white, sometimes tinged within with rose. 



Cliama gigas, LinniPus, Syst. Nat. Gmel. edit. p. 3299. 

 Tridacna gigas and mutica, Lamarck. 



Ilab. Indian and Pacific seas. 



This colossal species, which attains the enormous size 

 of from six to seven hundred pounds weight, may be dis- 

 tinguished in all its stages of growth by a tendency to 

 radiate anteriorly in an oblique direction, and by the 

 closely-scaled character of the ribs. The ribs are broad, 

 oidy four in number, and as the shell increases in age the 

 scales become shorter, closer-set, and more obscure. In 

 the shell represented in the opposite plate the interior is 

 tinged with deep rose-colour. 



June, lSfi2. 



