3ob MANUAL OF THE MOLLUSC A. 



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. 



Etijm. Plakous a thin cake. Ex. P. sella, PI. XVI. fig. 5. 



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

 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 distinc' 

 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 laminse, peculiarly separable, and occasionally penetrated by minute 

 tubuli. (Carpenter.) P. sella, called, from its shape, the "saddle-oyster, 3 

 is remarkably striated. In P. placenta, PL XVI. fig. 6, the anterior carti 

 lage ridge is only half so long as the other, which appears to be connecte< 

 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. 



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

 Syn. Hemiplacuna, G. Shy. Type, C. placunoides, 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. Distr. 3 sp. (Brit. Mus.) Tertiary, Egypt, America ? 



Placunopsis, Morris and Lycett. P. Jurensis, Rcemer. 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. F. Miffler. Scallop. 



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

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

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

 mented with radiating ribs; beaks approximate, eared; anterior ears 

 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- 



