414 MANUAL OF THE MOLLUSOA. 
Synonyms, Dianchora, Sby. Podopsis, Lam. Pachytes, Defr. 
Shell irregular, attached by the right valve, radiately ribbed, 
spiny or foliaceous; umbones remote, eared; lower valve with 
a triangular hinge-area, cartilage in a central groove, nearly or 
quite covered; hinge of two curved interlocking teeth in each 
valve ; adductor impression double. 
Animal, with the mantle open and gills separate, as in Pecten ; 
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 S. 
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 Dianchore. Specimens from the Miocene of St. 
Domingo, which have lost this layer, contain a loose mould of 
the original interior. Water-cayities are common in the inner 
layer, the border of the mantle haying 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, Pl. XVI., 
Fig. 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 ; Red Sea, Indian 
Ocean, Mauritius, Chinese Seas. 
PLICATULA, Lamarck. 
Etymology, plicatus, plaited. 
Type, P. cristata, Pl. XVI., Fig. 17 
Shell irregular, attached by the umbo of the right valve; 
valve smooth or plaited; hinge-area obscure ; cartilage quite 
internal; hinge-teeth, two in each valve; adductor scar simple. 
Animal resembles spondylus. 
Distribution, 9 species. West Indies, India, Philippines, 
Australia, West America. 
Fossil, 40 species. Trias—. United States, Europe, Algeria, 
india. 
P. Mantelli (Lea) Alabama, has the valves eared, 
