TRIBE NAYADES 7k 
IRIDINA. 
ENquivalve, inequilateral, transverse; beak small, turned to one side 
and nearly straight ; two separate lateral muscular impressions ; 
hinge long, linear, and attenuated near the centre, tuberculous and 
nearly crenated ; tubercles unequal ; ligament external and mar- 
ginal. 
I. Exotica Lam. 1—Bl. t. 66. f. 3.—E. t. 204. bis. f. 1.—W. 
Nate 2o2f. 10. Transversely elongated oblong, finely striated lon- 
gitudinally, both extremities rounded, the ventral edge sinuated in 
the middle; beaks but slightly projecting above. the margin. 
Africa. 
I. Rusens Desh. E. vol. 2. p. 320.—Anoponta R. Lam. 6.— 
E. t. 201. f.1 Rounded ovate, thick, reddish under a brown epi- 
dermis ; hinge crenated with obsolete transverse grooves : the 
sinus of the hinge-margin forming a deep and acute angle. —WNile 2 
I. Nizotica. Ferrussac. § Sow. Z. J. 1. t. 2—W.S. t. 2. Sf. = 
Myrizvus Dusius. Gmel.—D. p.118 Transversely elongated, very 
inequilateral, posteriorly attenuated,anteriorly produced and broader, 
cuticle greenish : hinge margin nearly straight, very slightly curved, 
angulated posteriorly ; toothless but with obsolete irregular undu- 
lations beneath the umbones: ventral edge slightly incurved sub- 
centrally: nacre purplish red.— Senaar, 2. .54 
I. Etoneata. Sow. G.—Reeve. t. 92.—Differs from Nilotica 
chiefly in having its hmge-margin strongly crenated, and its nacre 
brilliant and pure white. Senegal. 
I. Ovara. Swainson. Phil. Mag. 61.—Reeve. t. 93.—W. 8. t. 2. 
J. 9.—Piemon Macmurtret Conrad in Journ A. N. S. Phil. 7. 
t. 13. Elongated-ovate, strong, convex, ventral edge much ar- 
cuated, sides rounded, the anterior the broader and longer ; cuticle 
dark olive with obscure dark bluish green radiating lines, beaks 
usually eroded; nacre irridescent, silvery but often tinged with 
pink ; hinge margin subarcuated, becoming broad at the extremities, 
furnished with large tubercular crenations. 22..35. Liberia. 
I. Caxestis. Leaon U. 2. p. 82. t. 22. f. 70 Elongated, sub- 
cylindrical, very smooth, very inequilateral, thin, straight on the 
dorsal margin, angular before and rounded behind : beaks somewhat 
prominent and apparently without undulations at the tip; cuticle 
polished, very dark, nacre bluish and irridescent. 151,..32 Africa, 
Margins subparalkl. 

TRIBE CHAMACEA. 
Shell inequivalve, irregular, and adherent ; hinge with one tooth at 
most, and with two distinct lateral muscular impressions. 
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