410 MANUAL OF THE MOLLrSCA. 



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 Placunae are very closely allied to Anomia ; and many 

 intermediate forms may be traced. The shell of each consists 

 entirely of sub-nacreous, plicated laminae, 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 imnression below the-, 

 cartilage grooves of the upper valve, and a shallow sinus in theM 

 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. place7ita {VI. XYI., Fig. 6) we had predicated from examina- 

 tion of the shell, f The small muscular impressions before 

 and in the rear of the adductor are produced by suspensors 

 of the gills. 



Distribution, 4 species. Scinde, North Australia, China. 



Sub-genera. Carolia, Cantraine, 1835 (after Prince Charles] 

 Bonaparte). Synonym, Hemiplacuna, G. Sby. Type, C. placu-j 

 noides, PI. 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, Eoemer. 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, Europe. 



Placenta, Eetzius. 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. 

 t This organ appears to represent the byssal-sheath of Anomia, rather tlian the foot, 

 as tliere is no other opening for the passage of a bys&us. 



