26 MOLLUSCA. 



and it is only later, from the third year onwards, that they become females 

 an i.l produce ova. Moebius. on the contrary, asserts that the sperm is the later 

 formed, and not until after the pregnant beast has got rid of her eggs. The re- 

 production takes place especially in the months of June and July, at which 

 time, in spite of their extraordinary fertility, the oysters must not be gathered. 

 0. crixta. ijaUi Ohemn., in the Indian Ocean. A/tumia epliippntm L. Pltii-mni 

 placenta L. 



Fam. Pectiniiae. Scollops. Shell equivalved or unequivalved, but tolerably 

 cquisided, with straight hinge line ; often with fan-shaped ribs and bauds, with 

 single adductor muscle. The free and completely split mantle edges bear 

 numerous tentacles, and often emerald green eyes in great number. The small 

 foot often secretes byssus fibres for attachment. Some are attached by their 

 arched shell valve (Spondylns). others swim about by rapidly opening and 

 closing the shell (Prcten). Many are edible and are even more esteemed than 



Br 



FIG. 504. Mi/til us tiliilh (regne animal). O, mouth; s, labial palps; P, foot; S, byssus 

 secretion ; Br, gills ; M , thickened edge of mantle. 



i 



the oysters. Pecten Jacoba-iis L., P. ma.i-imiix L.. P. varinx L. Mediterranean. 

 Spondylus gaederopiis L. Lima sqiianioxtt Lam. 



Fam. Aviculidae. With oblique unequivalved shell of laminated texture and 

 inner mother-of-pearl layer. They possess two adductor muscles, of which, 

 however, the anterior is very small. Mantle completely open. Foot small, 

 secreting byssus. Ariciila hit-undo L.. Gulf of Tarent. Meleagrina margari- 

 tifera L., pearl mussel, inhabits especially the Indian and Persian Oceans, and 

 also the Gulf of Mexico. Secretes pearls.* The internal layer of the shell is used 

 in commerce as mother-of-pearl. Malleus mli/nrix, Lam., Indian Ocean. 



Fam. Mytilidae (fig. 504). Mussels. Shell equivalved, covered with 

 thick epidermis, with large posterior and small anterior muscle impressions. 

 The tongue- shaped foot fastens itself by the byssus fibres which it secretes. 

 Mantle more or less free except a short siphonal opening fringed at the edge. 

 Pinna xyuaniosa Gm.. Mediterranean. JUi/tilux rditHx I,., edible mussel of the 

 Xorth Sea and Baltic (fig. 504). Lithmlomiix dticft/lux Sow., in the Mediter- 



* cf. Moebius. " Die echten Perlen, etc."' Hamburg, 1857. 



