416 MANUAL OF THE MOLLUSCA. 
Distribution, 25 species. Mexico, South Britain, Mediter- 
ranean, India, Pacific :—20 fathoms. 
Fossil, 300 species. Lower Silurian —. World-wide. 
Sub-genera Maleagrina, Lam. Margaritophora, Muhlfeldt. 
M. margaritifera, Pl. XVI., Fig. 19. The “‘ pearl-oysters” are 
less oblique than the other avicule, and their valves are flatter 
and nearly equal; the posterior pedal impression is blended 
with that of the great adductor. They are found at Madagascar, 
Ceylon, Swan River, Panama, &c. Manillais the chief port to 
which they are taken. There are three principal kinds, which are 
worth from £2 to £4 per cwt. :—1. The silyer-lipped, from the 
Society Islands, of which about twenty tons are annually im- 
ported to Liverpool. 2. The black-lipped, from Manilla, of 
which thirty tons were imported in 1851. 3. A smaller sort 
from Panama, 200 tons of which are annually imported; in 
1851 a single vessel brought 340 tons. (T.C. Archer.) These 
shells afford the ‘‘ mother-o’-pearl”’ used for ornamental pur- 
poses ; and the ‘‘ oriental” pearls of commerce (p. 30, 31). Mr. 
Hope’s pearl, said to be the largest known, measures 2 inches 
long, 4 round, and weighs 1,800 grains. Pearl-oysters are found 
in about 12 fathom water; the fisheries of the Persian Gulf and 
Ceylon have been celebrated from the time of Pliny. ~ 
Malleus, Lam. M. vulgaris, Pl. XVI., Fig. 20. The ‘‘hammer- 
oyster’ is remarkable for its form, which becomes extremely 
elongated with age; both ears are long, and the umbones 
central. When young it is like an ordinary Avicula, with a 
deep byssal notch in the right valve. 6 species. China, 
Australia. 
Vulsella, Lam. V. lingulata, Pl. XVI., Fig. 21. Synonym, 
Reniella, Sw. Shell, oblong, striated, sub-equivalve ; umbones 
straight, earless. Often found imbedded in living sponges. 
Distribution, 7 species. Red Sea, India, Australia, Tasmania. 
Fossil, 7 species. U. Chalk —. Britain, France. 
Pteroperna, Lycett, 1852. P. costatula, Desl. Shell with a 
long posterior wing; hinge-line bordered by a groove; anterior 
teeth numerous, minute; posterior one or two, long, nearly 
parallel with the hinge-margin. /ossi/,3 species. Bath oolite ; 
Britain, France. 
? Aucella (Pallasu), 1846. Very inequivalve; left umbo pro- 
minent, earless; right valve small and flat, with a deep sinus 
beneath the small anterior ear. Jossil, 4 species, Permian — 
Gault. Europe. ‘In A. cygnipes we find no trace of prismatic 
cellular structure or nacre, but the coarsely corrugated and 
somewhat tubular structure of the Pectens.”” (Carpenter.) 
