MOLLUSCA: LAMELLIBRANCHIATA. 403 



surfaces of the valves, one being placed anteriorly in front of 

 the mouth, the other posteriorly on the neural side of the in- 

 testine. In the monomyary bivalves the posterior adductor 

 is the one which remains, and the anterior adductor is absent. 

 The adductors leave distinct " muscular impressions " in the 

 interior of the shell, so that it is easy to determine whether 

 there has been one only in any given specimen, or whether 

 two were present. 



The habits of the Lamellibranchiata are very various. Some, 

 such as the Oyster (Ostrea), and the Scallop (Pecleri), habitually 

 lie on one side, the lower valve being the deepest, and the 

 foot being wanting, or rudimentary. The former is fixed by 

 the substance of the valve, but the latter swims by rapidly 

 opening and closing the shell. Others, such as the Mussel 

 (Mytilus) and the Pinna, are attached to some foreign object 

 by an apparatus of threads, which is called the " byssus," and 

 is secreted by a special gland. Others are fixed to some solid 

 body by the substance of one of the valves. Many, such as 

 the Afyas, spend their existence sunk in the sand of the sea- 

 shore or in the mud of estuaries. Others, as the Pholades and 

 Lithodomi, bore holes in rock or wood, in which they live. 

 Finally, many are permanently free and locomotive. 



The Lamellibranchiata may be divided into two sections, 

 according as respiratory siphons are absent or present, as fol- 

 lows : 



SECTION A. ASIPHONIDA. Animal without respiratory 

 siphons; mantle -lobes free; the pallial line simple and not 

 indented (Integropallialia). 



This section comprises the families Ostreidce, Aviailida, 

 Mytilidce, Arcadce, Trigoniadce, and Uniomdce. 



The Ostreidce (including the Oysters, Scallops, Anomice, Thorny Oysters, 

 &c. ) are all marine, and are monomyary. The Aviculida, or Pearl-oysters, 

 are likewise marine, but are dimyary. The Mytilidce (Mussels, Horse- 

 mussels, &c.) are partially marine and partially fresh- water forms, and 

 have a very small anterior adductor. The Arcadce (Ark-shells, &c.) arc 

 exclusively marine, as are the nearly allied Trigoniadcz. Lastly, the Union- 

 idee (Fresh-water Mussels) are exclusively confined to rivers and lakes. 



SECTION B. SIPHONIDA. Animal with respiratory siphons ; 

 mantle-lobes more or less united. 



Two subdivisions, of little classificatory or anatomical value, 

 are comprised in this section. In the first the siphons are short, 

 and the pallial line is simple (Integropallialia} ; as is seen in 

 the families Chamida, Hippuritidce, Tridacnidce, Cardiada, 

 LurinidcB, Cydadidce, and Cyprinidcz. 



The second subdivision (Sinupallialia) is distinguished by 



