CLASSIFICATION OF LAMELLIBRANCHIATA. *395 



is unsatisfactory (1016). By others, the degree in which the 

 two divisions, or lobes of the mantle, are united along their edge, 

 is adopted as the foundation of the arrangement ; this also is 

 unsatisfactory for a similar reason, although it is probably a more 

 natural character than the other, because it seems to correspond 

 more with the general structure of the animal. Others, again, 

 have taken the degree of development of the foot as their guide ; 

 but this, too, if followed alone, would lead into many errors. 

 The fact appears to be, that it is necessary to consider all these 

 characters together, in attempting to make a natural arrangement 

 of this family ; and the Author's recent inquiries regarding the 

 structure of the Shell, lead him to believe that this also will 

 afford a character of great importance, frequently serving to 

 determine the real position of genera, which would otherwise be 

 doubtful. Most of recent writers on this .branch of Zoology, 

 have adopted a character nearly coincident with the second of 

 those referred to above, namely, the presence or absence of 

 siphons, as the means of establishing two great primary di- 

 visions in this Class. The Orders thus formed, the SIPHONATA 

 and the ASIPHONATA, may be again subdivided into numerous 

 groups distinguished by characters of importance; those here 

 adopted nearly correspond with the families admitted by M. de 

 Blainville. 



I. The ASIPHONATA, in which the two sides of the mantle are 

 separated, or but slightly adherent at some part of their edges, 

 and in which there is no trace of a siphon or any analogous 

 formation, may be divided into six Sections ; namely : 



A. The OSTRACE.E, including the Oysters and their allies. 

 These have the lobes of the mantle open along their whole length, 

 but the body is concealed by the adhesion of the laminae of the 

 gills ; the foot is altogether absent ; and there is but a single 

 adductor muscle. 



B. The PECTINID^E, or Pectens (Scallop-shells), and their 

 allies, which have, like the last, the lobes of the mantle open ; 

 but the branchial laminae are not adherent ; there is but a single 

 adductor muscle ; and there is usually the rudiment of a foot. 



