17 
Ir1p1na.— 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. pl. 2.) (Mr. G. B. Sowerby’s Cabinet.] 
CHAMACEA._—_3 Genera. 
Shell inequivalve, irregular, fixed; hinge mith one thick tooth, or without teeth: two separate and 
lateral muscular impressions. 
Dicrras.—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,] 
Cuama.—Shell irregular, inequivalve, fixed; the beaks curved and unequal: hinge with 
one thick, oblique, subcrenate tooth, fitting in a pit in the opposite yalve; 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. 
ErueriA.—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), (0). 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. ] 
(6) Outside of upper valve of ditto, 
