TRIGONIA 



Plate I. 



Genus TRIGONIA, Bruguiere. 



Testa aquivalvis, oblique trigono-ovata, inlus vivide irides- 

 cente-margaritacea, roseo-purpurascens vel argentea 

 vel aurea, extus radiatim nodoso-costata, violaceo-pur- 

 pitrea, epidermide fused induta. Cardo dentibus oblon- 

 gis, lamelliformibus, divaricatis, in valvd dextrd duobus, 

 utroque latere transversim sulcatis, in valvd sinistra 

 quatuor, uno latere tantum sulcato. Ligamentum ex- 

 ternum, marginale. 



Shell equivalve, obliquely triangularly ovate, bright irides- 

 cent-pearl within, rose-purple, silvery or golden, ex- 

 ternally radiately nodosely ribbed, violet-purple, co- 

 vered with a brown epidermis. Hinge composed of 

 two divaricate, oblong, lamelliform teeth, transversely 

 grooved on both sides in the right valve, interlocking 

 between four similar teeth, transversely grooved only 

 on the inner side, in the left valve. Ligament exter- 

 nal, marginal. 



Of the genus Trigonia more than a hundred fossil 

 species are known from different parts of Europe, Asia, 

 South Africa, and even south-western America ; but it only 

 appears in a living state on the shores of Australia and 

 Tasmania. Six recent species have been described, and 

 they may be reduced to the four represented in our 

 Plate. 



The shell of Trigonia has of all bivalves the most bril- 

 liantly iridescent nacre in the interior ; and it has the 

 most closely interlocking hinge. The shell-collector prides 

 himself largely on the display which the pearly lining of 

 the valves, exquisitely rose-purple or orange-tinted, with the 

 addition of a silvery or golden hue, makes in his cabinet ; 

 and when separated it requires no little ingenuity and 

 neatness of handling to close them again. In the right 

 valve are two large, oblong, divaricate teeth, transversely 

 grooved on each side, and these fit closely between two 

 divaricate pairs of similarly grooved teeth in the left valve, 

 grooved only on the inner side where the teeth of the cor- 

 responding valve are received. 



The animal of Trigonia, described originally by MM. 

 Quoy and Gaimard, and afterwards by Professor Huxley, 

 has the edge of the mantle crimped to fill the hollows of 

 the ribs of the shell, and the foot is sharply angled and 

 crenated, possessing considerable muscular action. Mr. 

 Samuel Slutehbury, while dredging iu Syduey Harbour, 



placed a Trigonia on the gunwale of his boat, and it leaped 

 overboard, clearing a ledge of four inches. 



Species 1. (Pig. a, b, c, Mus. Cuming.) 



Trigonia Lamarckii. Trig, testa subventricosd, solidd, 



antice subobtuso-angulatd, coslis confertiusculis, muri- 



calo-nodatis. 

 Lamarck's Trigonia. Shell somewhat veutricose, solid, 



anteriorly somewhat obtusely angled, ribs rather 



close-set, prickly-noduled. 

 Trigonia Jukesii, Adams. 

 Hob. Australia. 



In this species, which is the most abundant of the 

 genus, the shell is stouter and more quadrately ventricose 

 than the rest. The ribs are narrower and closer, and the 

 nodules incline more to the form of prickly scales. Fig. 

 1 a represents a specimen in which the pearly nacre of the 

 interior is of a golden or salmon-colour, and Fig. 1 b and 

 Fig. 1 c varieties of coarser and finer varieties of sculp- 

 ture. T. Jukesii appears to me to be a specimen of the 

 coarser variety. 



Species 2. (Fig. a, b, Mus. Cuming.) 

 Trigonia uniophora. Trig, testa couipressiusculd, pos- 

 tice angulation compressi expanse!, coslis subdistantibus, 

 nodis rotundatis, obtusis. 

 The plough-share Trigonia. Shell rather compressed, 

 angularly compressly expanded posteriorly, ribs ra- 

 ther distant, nodules rounded, obtuse. 

 Gray, App. to Jukes' Narr. Toy. Fly, p. 361. pi. 2. f. 5. 

 Hab. Cape York, Australia (in six fathoms); Jukes. 



Of this species Mr. Cuming possesses only the spe- 

 cimens Fig. 2 b and the odd valve Fig. 2 a which is the 

 more characteristic of the two. It is of a marked ob- 

 liquely expanded, compressed growth, with fewer, more 

 distant ribs, on which the nodules are rounded and ob- 

 tuse. 



Species 3. (Fig. a, b, c, d, Mus. Cuming.) 



Trigonia margaritacea. Trig, testa oblique suborbi- 

 culari, modice convexd, postice parum angulatd, coslis 

 minus proniinentibus, distantibus, nodis parviusculis. 



The pearly Trigonia. Shell obliquely suborbicular, 



September, 1860. 



