17 
Iridina. — Shell equivalve, inequilateral, transverse; the beaks small, reflected, almost 
straight ; muscular impressions similar to the genus Anodon ; hinge long, linear, at- 
tenuated about the middle, tuberculated throughout the whole length, almost cre- 
nated; the tubercles unequal, frequent. Ligament external, marginal. 
Plate X. Fig. 1. I. Nilotica. (Zoological Journal, vol. i. pi. 2.) [Mr. G. B. Sowerby’s Cabinet.] 
CHAMACEA 3 Genera. 
Shell inequivalve, irregular, fixed; hinge with one thick tooth, or without teeth : two separate and 
lateral muscular impressions. 
Diceras. — Shell inequivalve, adhering; the beaks conical, very large, diverging, irregu- 
larly spiral; one large, thick, concave, subauricular, prominent tooth in the greater 
valve : two muscular impressions. 
Plate XXI. Fig 3. D. arietina. — Fossil. [Mr. J. D. C. Sowerby’s Cabinet.] 
Chama. — Shell irregular, inequivalve, fixed; the beaks curved and unequal: hinge with 
one thick, oblique, subcrenate tooth, fitting in a pit in the opposite valve; two dis- 
tant, lateral, muscular impressions. Ligament external, sunk. 
(1) Shells whose beaks turn from the left to the right. 
Plate X. Fig. 2. C. Lazarus. (Idem — Linn.) 
(2) Shells whose beaks turn from the right to the left „ 
Plate X. Fig. 3. C. radians. 
Etheria. — Shell irregular, inequivalve, adhering; the beaks short, almost sunk in the 
base of the valves : hinge without teeth, rather sinuous, unequal ; two distant, la- 
teral, oblong muscular impressions. Ligament external, tortuous, partly pene- 
trating the shell. 
(1) Shells having an oblong callosity in their base. 
Plate X. Fig. 4 (a), (b). E. elliptica. [British Museum.] 
(2) Shells without the callosity in their base. 
Plate X. Fig. 5. (a) Inside of E. semilunata. [Mr. J. D. C. Sowerby’s Cabinet.] 
( h ) Outside of upper valve of ditto. 
D 
