26 MOLLUSCA. 



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

 and 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 should not be gathered. 

 0, crista galli Chemn., in the Indian Ocean. Anomia eplilppium L. Placuna 

 placenta L. 



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

 equisided, with straight hinge line ; often with fan-shaped ribs and bands, 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 (Sjjondylus), others swiin about by rapidly opening and 

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



Br 



Fie. 804.Mytilus edulis (regne animal). O, mouth ; /S, labial palps ; P, foot ; S, byssus 

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



the oysters. Pecten Jacobceus L., P. maximus L., P. varius L. Mediterranean. 

 Spondylus gaederopus L. Lima squamosa 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. Avinda hirundo L., Gulf of Tarentum. Meleagrina marga- 

 ritifera 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 vulgar is.'La.m., 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 squamosa Gm., Mediterranean. Mytilus edulis L., edible mussel of the 

 North Sea and Baltic (fig. 504). Litlunlomus dactylus Sow., in the Mediter- 



* of. Moebius, " Die echten Perlen, etc." Hamburg. 1857 



