119 



Tlie disk of the upper valve with three separate subcircular scars ; 

 the two upper scars small, subequal, one under the other ; the lower 

 one large, nearly circular, subceutral. Notch iu lower valve very 

 small. Plug small, elongate, subcylindrical ; the notch small, with 

 reflexed edges. 



Hub. Port Essington ; Earl of Derby. Depuch Island ; Capt. Sir 

 Everard Home, Bart. British Museum. 



Var. 1.? Shell very thin. Mus. Cuming. 



Var. 2. Very thick ; disk white, very thick. Mus. Cuming. 



The small size of the upper scars in this species probably depends 

 on the small size and elongated form of the plug. The other species, 

 which have the upper scar the largest, have at the same time a larger 

 notch and a broader plug. 



II. Placunanomia. 



Upper or dorsal valve with two subcentral muscular scars ; the 

 upper scar radiately veined. Byssal notch distinct, converted into a 

 hole by the upper part of the anterior lobe of the notch being sol- 

 dered to and forming part of the cardinal edge : the plug triangular, 

 gradually enlarging in size ; the apex and outer surface next to the 

 body to which it is attached, calcareous, longitudinally striated ; the 

 inner surface covered with horny, longitudinal, parallel laminae, and 

 more or less agglutinated to the edge of the notch. 



Syn. Placunanomia, Broderip, Proc. Zool. Sac. 1832, 29 ; Miiller, 

 Syn. 176 ; Besh. in Lamk. Hist. vii. 269. 



Anomia, ft, Schumacher, Essai, 1817. 



Anomia, pars, Blainv. Man. Moll. ; Montague ; Forbes ^ Hanley. 



Ostrea, sp. Da Costa ; Montague. 



Placunonomia, D'Orb. Am6r. Mcrid. 



Placunomia, Swains. Malac. 39, 1840. 



Pododesmus, Philippi, Wieymann Arch. i. 385, 1837- 



Mr. Broderip, who established this genus, does not observe the 

 character furnished by the muscular impressions, or the lobe of the 

 notch : he merely says, " Impressio muscularis in utraque valvfi sub- 

 centralis. In valva superiore organi adhesionis impressio superad- 

 dita." And further, that " the organ of adhesion, which in its bony 

 character (for it is more bone than shell) resembles that of Anomia, 

 docs not perforate the lower valve directly, but is inserted between 

 the laminae of the internal surface of the lower valve, above the mus- 

 cular impression and below the hinge, and passes out into an ex- 

 ternal, irregular, somewhat longitudinal, superficial fissure or cica- 

 trix, which is narrowest at the hinge margin, and which it entirely 

 fills to a level with the surrounding surface." 



This form is produced by the gradual increase of the size of the 

 plug and the simultaneous increase of the size of the shell. 



Some have considered the "plug" or "stopper" oi Anomia to be a 

 third valve, which is evidently a mistake. Philippi {Moll. Sicil. i. 92) 

 considers it as the ossification of the tendon of the adductor muscle. 



Mr. Broderip, in the passage cpioted, regards it as a bone. In Dr. 

 Dieffenbach's Travels I have remarked : " The plug is evidently only 



