PLACENTAD^. 
O 
This shell forms the passage between the genus Anomia, or rather 
Placunanomia, and Placuna. It shows the gradual change which 
takes place between the three genera. In Anomia there are two 
muscles for the purpose of attaching itself to marine bodies, which 
form a plug which is free from the sinus of the shell. In Placu- 
nanomia there is only a single muscle to perform the same office, 
and in the more typical species of this genus the plug itself is fixed 
into the surface of the shell, forming, as it were, part of its substance. 
In Hemiplacuna and Placuna there is no muscle or plug for attach- 
ment, and the shells are free; but in Hemiplacuna there is a rudi- 
mentary development of the sinus through which the plug is usu- 
ally emitted, and the ligament which connects the shell is of the 
same form as that found in the genera Anomia and Placunanomia. 
The name for the genus is not consistent with the Linnasan ca- 
non ; but used rather than burthen the genus with two names. 
1. Hemiplacuna Rozieei. 
Placuna, .sp., Roziere, Description d’Egypte, Miniralogie, t. 11, f. 6. 
Hemiplacuna Rozieri, G. B. Sotv. MSS. ; Gray, P. Z. S. 1849, 
124. 
Anomia? or Placuna? Desk, in Lamk. Hist. vii. 270, note. 
a. Fossil. Shore of the Red Sea ; Vallee de I’Egarement. 
Purchased of Mr. Sowerby. 
