416 MANUAL OF THE MOLLUSCA. 



Distrihution, 25 species. Mexico, South Britain, Mediter- 

 ranean, India, Pacific : — 20 fatlionis. 



Fossil, 300 species. Lower Silurian — . World-wide. 



Sub-genera 3Ialeagrina. Lam. Margaritophora, Muhlfeldt. 

 M. margaritifera, PI. XYL, Fig. 19. The "pearl-oysters" are 

 less oblique than the other aviculce, 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 Eiver, Panama, &c. Manilla is the chief port to 

 which they are taken. There are three principal kinds, which are 

 worth from £2 to £4 per cwt. : — 1. The silver-lipi)ed, from the 

 Society Islands, of which about twenty tons are annually im- 

 ported to Liverpool. 2. The black-lijii^ed, 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. 0. Archer.) These 

 shells afford the " mother-o' -pearl" used for ornamental pur- 

 poses ; and the " oriental" jDearls 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 XYL, 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. Y. lingulata, PI. XYL, Fig. 21. Sijnomjm, 

 Eeniella, Sw. Shell, oblong, striated, sub-equivalve; umbones 

 straight, earless. Often found imbedded _ in living sjionges. 

 Distrihution, 7 species. Eed 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. Fossil, 3 species. Bath oolite ; 

 Britain, France. 



? Aucella (Pallasii), 1846. Yery inequivalve ; left umbo pro- 

 minent, earless ; right valve small and flat, with a deep sinus 

 beneath the small anterior ear. Fossil, 4 species, Permian — 

 Gault. Europe. " In ^: cygnij^es we find no trace of prismatic 

 cellular structure or nacre, but the coarsely corrugated and 

 somewhat tubular structure of the Pectens." (Carpenter.) 



