414 MAJTUAL OF THE MOLLUSCA. 



Synonyms, Diancliora, Sby. Podopsis, Lam. Pachytes, Defr. 



Shell irregular, attached by tlie right valve, radiately ribbed, 

 spiny or foliaceous ; -umbones remote, eared ; lower valve with 

 a triangular binge-area, cartilage in a central groove, nearly or 

 quite covered ; binge of two curved interlocking teetb in each 

 valve ; adductor impression double. 



Animal, with the mantle open and gills separate, as in Peden; 

 lips foliaceous, palpi short ; foot small, cylindrical, truncated. 



In aged specimens the circular portion of the muscular scar 

 exhibits dendritic vascular markings. The lower valve is 

 always most spiny and least coloured ; in some species (like 8. 

 imperialis) the shell is scarcely, if at all, attached by its beak 

 or spines. The inner shell-layer is very distinct from the outer, 

 and always wanting in fossil specimens from calcareous rocks, 

 then called Dianchoroe. Specimens from the Miocene of St. 

 Domingo, which have lost this layer, contain a loose mould of 

 the original interior. Water- cavities are common in the inner 

 layer, the border of the mantle having deposited shell more 

 rapidly than the umbonal portion. (Owen, Mag. Nat. Hist. 

 1838, p. 409.) 



Distribution, 68 species. "West Indies, Canaries, Mediter- 

 ranean, India, Torres Straits, Pacific, West America : — 105 

 fathoms. 



Fossil, 80 species. Carb — . Europe, United States, India. 



Sub-genus, Pedum, Brug. P. spondyloides, PI. XYI., 

 Pig. 16. Shell thin, smooth, compressed, attached by a byssus 

 passing through a deep notch in the right valve. Inhabits 

 coral-reefs, where it is found half-embedded ; Eed Sea, Indian 

 Ocean, Mauritius, Chinese Seas. 



Plicattjla, Lamarck. 



Etymology, plicatus, plaited. 



Type, P. cristata, PL XYL, Pig. 11 



Shell irregular, attached by the umbo of the right valve, 

 valve smooth qr plaited ; hinge-area obscure ; cartilage quit 

 internal ; hinge-teeth, two in each valve ; adductor scar simpl^ 



Animal resembles spondylus. 



Distribution, 9 species. West Indies, India, Philippine 

 Australia, West America. 



Fossil, 40 species. Trias — . United States, Europe, Algeija, 

 India. 



P. Mantelli (Lea) Alabama, has the valves eared. 



