123 
ttt Australian. 
9. PLACUNANOMIA ZEALANDICA. Sym ' 
Suborbicular, white, smooth ; upper valve with distant radiating 
grooves ; internally dark green. 
Upper valve with two confluent scars ; upper oblong, longitudinal, 
lower rather small and more transverse. 
Anomia Zealandica, Gray, in Dieffenbach’s New Zealand, ii. 261, 
1843. 
Hab. New Zealand ; on the inside of mussel shells. 
10. PLACUNANOMIA IONE. 
Shell white, laminar ; edge of the laminz with small, slender, elon- #4, 
gated processes ; internally green. 1 
Lower muscular scars small, round, on the lower hinder edge of 
the larger one; sinus or perforations large. 
. = Hab. Australia, Sydney; on rocks, Mr. Strange. 
Mus. Cuming; three specimens. ? Van Diemen’s Land. By 
Pym . — Dr. Sinclair, Brit. Mus., a single dorsal valve. 
11. PLACUNANOMIA COLON. 
- Shell (upper valve) flat, with rather irregular, flat, radiating ribs ; 
white, lower spotted ; upper valve with two separate scars ; the upper 
one oblong, longitudinal, the lower much smaller, circular. 
Hab. ? 
Mr. Cuming’s Collection (no. 10). Mr. Humphrey’s Collection ; 
a single upper valve of a rather young shell. 
Here may be added the description of a new genus, intermediate 
between this family and Placunide. 
TI. Hemrpevacuna. 
Shell free ; valves orbicular, flat, external surface minutely laminar 
and radiately striated, especially on the edge of the plates; muscular 
scar in each valve single, nearly central, circular ; the right valve flat, 
with a large oblong, elevated transverse process for the cartilage, 
haying a very small concavity in the inner surface in front of the car- 
tilaged process representing the sinus in dnomia ; the left valve rather 
more convex, with an oblong transverse pit for the internal cartilage 
under the umbo. 
Hemiplacuna, G. B. Sowerby, MSS. 
This shell has all the external characters of the flat species of Pla- 
cuna, and has the same muscular impression ; but instead of having 
the two linear diverging ridges and grooves to give attachment 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 imner surface of the right valve, just in 
front of the base of the process which supports the cartilages, there 
is a small shallow roundish pit with a short furrow towards the centre 
of the shell, which is evidently a rudimentary representation of the 
sinus found in the genus dnomia. This sinus is not visible on the 
outer surface of the shell. 
