CONCHIFERA. 409 



pearly within, attached by a ping passing through a hole or 

 notch in the right valve : upper valve convex, smooth, lamellar 

 or striated ; interior with a sub-marginal cartilage-pit, and 

 four muscular impressions, 3 sub-central, and one in front of 

 the cartilage (see Fig. 213, p. 402) : lower valve concave, with a 

 deep, rounded notch in front of the cartilage process; disk with 

 a single (adductor) impression. 



Animal with the mantle open, its margins with, a short double 

 fringe ; lips membranous, elongated ; palpi fixed, striated on 

 both sides ; gills 2 on each, side, united posteriorly, the outer 

 laminae incomplete and free ; foot small, cylindrical, subsidiary 

 to a lamellar and more or less calcified byssal plug, attached to 

 the upper valve by three muscles ; adductor muscle behind the 

 byssal muscles, small, composed of two elements; sexes dis- 

 tinct ; ovary extending into the substance of the lower mantle- 

 lobe. 



In A. pemoides, from California, there is an anterior {pedal) 

 muscular impression in both valves. 



" There is no relationship of affinity between Anomia and 

 Terehratula, but only a resemblance through, formal analogy; 

 the parts which seem identical are not homologous." (Forbes.) 

 The Anomiae are found attached to oysters and other shells, 

 and frequently acquire the form of the surfaces with which 

 their growing margins are in contact. They are not edible. 



Distribution, 20 species. North America, Britain, Black Sea, 

 India, Australia, West America, Icy sea. Low water — 100 

 fathoms. 



Fossil, 36 species. Oolite — . Chili, United States, Europe, 

 India. 



Sub-genera. Placunomia (Cumingii), Broderip. Synonym, 

 Pododesmus, Phil. P. macroscliisma, PI. XYI., Pig. 4. Upper 

 valve with only two muscular impressions ; the pedal scar 

 radiately striated ; the byssal plug is often fixed in the lower 

 valve, and its muscle becomes (functionally) an adductor. Dis- 

 tribution, 13 species. West Indies, Britain (P. patelliformis), 

 New Zealand, California, Behi'ing's Sea, Ochotsk. — 50 

 fathoms. 



Limanomia (Grayana), Bouchard. Shell eared like Lima. 

 Fossil, 4 species. Devonian ; Boulonnais, China ? 



Plachna, Solander. — Window-Shell. 



Etymology, plahous, a thin cake. 

 Example, P. sella, PI. XVI., Fig. 5. 



Shell suborbicular, compressed, translucent, free, resting on 



T 



