692 MOLLUSC A. 



previously seen, the presence or absence of retractile siphons can be 

 readily determined merely by inspection of the dead shell. 



The Lamellibranchiata are wholly aquatic, living in either salt or 

 fresh water, and their habits are very various. Some, such as the 

 Oyster (Ostrea), and the Scallop (Pecten), 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 a " byssus." Others are fixed to some solid body 

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

 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 have often been divided into two primary 

 sections, according as " siphons " are or are not developed. In one 

 of these sections — Asifihonida — the mantle-lobes are free, and there 

 are no respiratory siphons, so that the pallial line is simple and is 

 not indented. In the other great section — Siphonida — the mantle- 

 lobes are more or less united, and respiratory siphons are present. 

 In certain groups of the Siphonate Bivalves the siphons are short, 

 without specially developed retractor-muscles, and the pallial line is 

 therefore simple, the name of Lntegropallialia being given to such 

 forms. On the other hand, in many types of the Siphonate Bivalves 

 — hence termed Sinupallialia — there are long siphons, the greatly 

 developed retractor-muscles of which are attached to an indenta- 

 tion or bay in the pallial line. This classification is doubtfully 

 natural, and is largely inapplicable to fossil forms ; for which reasons 

 the arrangement adopted by Dr Paul Fischer in his ' Manuel de 

 Conchyliologie ' has, in the main, been here followed in preference. 

 Owing, however, to the great importance in the study of the fossil 

 Bivalves of the various internal markings of the shell, it may be of 

 use here to indicate generally the principal groups of Lamellibranchs 

 which agree with one another in the general nature of the impres- 

 sions left by the pallial line and adductors ; a guide being thus given 

 to the families amongst which the affinities of a given fossil shell 

 must be sought. From this point of view, the Bivalves fall into the 

 four following sections, the families of which, though often allied, 

 have no necessary zoological relationship with one another : — 



(1.) The following families of Lamellibranchiata are " monomyary," 

 and have an " entire " pallial line, no siphons being developed — viz., the 

 Ostreida*, A?iomiida>, Sftoftdylidce, LimidcB, and Pectinidce. This group 

 of families corresponds with the Monomyaria of some writers. 



(2.) The following" families are " heteromyary," and have no siphons, the 



