356 MANUAL OF THE MOLLUSCA. 



in the lower valve, and its muscle becomes (functionally) an adductor. 

 Distr. 12 sp. W. Indies, Brit. {P. patelliformis). New Zealand, California, 

 Behring's sea, Ochotsk. — 50 fms. 



Limanomia (Grayana) Bouchard. Shell eared like Lima. Fossil, 4 sp. 

 Devonian ; Boulonnais, China ? 



Placuna, Solander. Window- shell. 



Etpn. Flakous a thin cake. Ex. P. sella, PI. XVI. fig. 5. 



Shell sub-orbicular, compressed, translucent, free, resting on the right 

 valve ; hinge area narrow and obscure ; cartilage supported by two diverging 

 ridges in the right valve and corresponding grooves in the left ; muscular 

 impressions double, the larger element round and central, the smaller distinct 

 and crescent shaped, in front of it. 



The Placunse are very closely allied to Anomia ; and many intermediate 

 forms may be traced. The shell of each consists entirely of sub-nacreous, 

 plicated laminas, peculiarly separable, and occasionally penetrated by minute 

 tubuli. {Carpenter.) F. sella, called, fi'om its shape, the "saddle-oyster," 

 is remarkably striated. In F. placenta, PI. XVI. fig. 6, the anterior carti- 

 lage ridge is only half so long as the other, w hich appears to be connected 

 with the economy of the shell when young ; in specimens 1 inch across, 

 there is a pedal impression below the cartilage grooves of the upper valve, 

 and a shallow sinus in the margin of the lower valve, indicating a slight 

 byssal attachment at that age. 



Distr. 4 sp. Scinde, N. Australia, China. 



Suh -genera. Carolia, Cantraine 1835, (after Prince Charles Bonaparte.) 

 Spi. Hemiplacuna, G. Sby. Type, C. placunoides, PI. XVI. fig. 7. Shell 

 like Flacuna ; 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. Distr. 3 sp. (Brit. Mus.) 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 ; ligamental groove sub-marginal, trans- 

 verse ; muscular impression large, sub-central. Fossil, 4 sp. Lower Oolites, 

 Europe. 



Pecten, 0. P. Miiller. ScaUop. 

 Etym. Fecten, a comb. Type, P. maximus (Janira, Schum.) 

 Syn. Argus, Poll. Discites, Schl. Amusium, Muhlfeldt. 

 Shell sub-orbicular, regular, resting on the right valve, usually orna- 

 mented with radiating ribs; beaks approximate, eared; anterior eai-s 

 most prominent ; posterior side a little oblique ; right valve most convex, 

 with a notch below the front ear; hinge-margins straight, united by a 

 narrow ligament; cartilage internal, in a central pit; adductor impres- 



