410 MANUAL OF THE MOLIx, SCA. 
the right valve; hinge area narrow and obscure; cartilage 
supported by two diverging ridges in the right valve and cor- 
responding grooves in the left; muscular impressions double, 
the larger element round and central, the smaller distinct and 
crescent shaped, in front of it. 
The Placune are very closely allied to Anomia; and many 
intermediate forms may be traced. ‘The shell of each consists 
entirely of sub-nacreous, plicated laminz, peculiarly separable, 
and occasionally penetrated by minute tubuli. (Carpenter.) P. 
sella, called, from its ‘shape, the ‘‘ saddle-oyster,” is remarkably 
striated. In P. placenta, Pl. XVI., Fig. 6, the anterior carti- 
lage ridge is only half as long as the other, which appears to be 
connected with the economy of the shell when young ; in speci- 
mens 1 inch across, there is a pedal imvression below the 
cartilage grooves of the upper valve, and a shallow sinus in the 
margin of the lower valve, indicating a slight byssal attach- © 
ment at that age. 
Placuna* is essentially like Anomia, having the generative © 
system attached to the right mantle-lobe, and the ventricle 
exposed. The mantle-margin is cirrated, and furnished with a 
curtain, as in Pecten; the foot is tubular and extensile, but has 
no distinct muscles except the small one, whose existence in 
P. placenta (Pl. XVI., Fig. 6) we had predicated from examina- 
tion of the shell.t The small muscular impressions before 
and in the rear of the adductor are produced by suspensors 
of the gitls. 
Distribution, 4 species. Scinde, North Australia, China. 
Sub-genera. Carolia, Cantraine, 1835 (after Prince Charles 
Bonaparte). Synonym, Hemiplacuna, G. Sby. Type, C. placu- 
‘ noides, Pl. XVI., Fig. 7. Shell like Placuna; hinge, when 
young, like Anomia, with a byssal plug passing through a 
small deep sinus in front of the cartilage process, which is closed 
in the adult. Distribution, 3 species. (British Museum), Tertiary, 
Egypt; America? 
Placunopsis, Morris and Lycett. P. Jurensis, Roemer. Sub- 
orbicular, upper valve convex, radiately striated, or taking the 
form of the surface to which it adheres; lower valve flat; liga- 
mental groove sub-marginal, transverse; muscular impression 
large, sub-central. fossil 4 species. Lower Oolites, Kurope. 
Placenta, Retzius. Cartilage grooves slightly divergent, the 
posterior one the longer of the two; muscular impression sub- 
central. 
* Original figures and descriptions will be found in the An. Nat. Hist. 1855, p. 22. 
+ This organ appears to represent the byssal-sheath of Anomia, rather than the foot, 
£5 there ‘s no other opening for the passage of a vyssus. 
