4 
CATALOGUE OF 
1. Placenta obiculaeis 
Shell colourless, semitransparent ; when young, pale purplish. 
Placuna placenta, Solander, MSS.; Lamk. Hist. N. 3. 
Anomia placenta, Linn. S. N. 1154 ; Ckemn. Conch, viii. t. 79, f. 
176, cop. E. M. t. 173, f. 2; Dillw. R. S. i. 297 ; Lister, Conch. 
t. 225, f. 60, t. 226, f. 61. 
Placenta orbicularis, Retz. Dissert. 15, 1788 ; Gray, P. Z. S. 
1848. 
a. Adult. China. 
b, c. Adult. China. Mus. Broderip. 
d. Nearly adult. India. Presented by Dr. Horsfield. 
Half grown. China. 
i,j. Small, rather thicker. N. E. coast of Australia or Port 
Essington. Presented by the Earl of Derby. 
b. Hinge-ridge transverse, elevated on a broad, high, oblique process. 
3. HEMIPLACUNA. 
Shell free ; valves orbicular, Bat, external surface minutely laminar 
and radiately striated, especially on the edge of the plates ; mus- 
cular scar in each valve single, nearly central, circular ; the right 
valve flat, with a large, oblong, elevated, transverse process for 
the cartilage, having a very small concavity in the inner surface 
in front of the cartilaged process representing the sinus in Ano- 
mia ; the left valve rather more convex, with aii oblong, trans- 
verse pit for the internal cartilage under the umbo. 
Hemiplacuna, G. B. Sowerby, MSS. ; Gray, P. Z. Soc. 1849, 
123. 
Anomia or Placuna sp. Desh. in Lamk. Hist. 2 ed. vii. 270, note. 
This shell has all the external characters of the flat species of 
Placuna, and has the same muscular impression ; but instead of 
having the two linear, diverging ridges and grooves to give attach- 
ment to the cardinal cartilage, it has an oblong, elevated process in 
the right valve, and an oblong cavity in the left, exactly similar to 
those found in the genus Anomia ; and on the inner surface of the 
right valve, just in front of the base of the process which supports 
the cartilage, there is a small, shallow, roundish pit, with a short 
furrow towards the centre of the shell, which is evidently a rudi- 
mentary representation of the sinus found in the genus Anomia. 
This rudimentary sinus is not visible on the outer surface of the 
shell. 
