^ 170. THE ACEPHALA. 185 



Genera : Orbicula, Terebratula, langvla. 



ORDER III. LAMELLIBRANCHIA. 



Animals which are symmetrical and bivalved, and whose more or less 

 closed mantle encloses two pairs of lamelliform tentacles and branchiae. 



SUB-ORDER I. MONOMYA. 



Family : Ostracea. 

 Genera : Ostrea, Atwmia. 



Family : Pectinea. 

 Genera : Pecten, Spondylus, Lima. 



Family : Malleacea. 

 Genera : Malleus, Perna, Crenatula. 



SUB-ORDER II. DIMYA. 



Family: Aviculacea. 

 Genera : Avicida, Meleagrina, Pinna. 



Family : Arcacea. 

 Genera : Area, Pectunculus, Trigo7iia, Nucula. 



Family ; Naiades. 

 Genera : Anodonta, Unio. 



Family : Mytilacea. 

 Genera : Mytilus, Modiola, Litkodomus, Tichogonia. 



Family : Chamacea. 

 Genera : Chama, Isocardia. 



Family : Cardiacea. 



Genera : Cardium, Lucina, Hiatella, Cyclas, Piscidium, Tellina, Psam- 

 mobia, Yenus, Cytherea, Venerupis, Mactra, Lutraria, Ungulina. 



Family : Pyloridae. 

 Genera : My a, Solen, Solenomya, Panopaea. 



SUB-ORDER III. INCLUSA. 



Family : Teredina. 

 Genera : Pholas, Teredo. 



