CLASSIFICATION OF LAMELLIBRANCHIATA. 395 



principally frequenting the shores or shallows. Each region has 

 certain species peculiar to it, or most abundant in it ; and there are 

 few which are not limited to one hemisphere. The temperate 

 zone appears as favourable to the development and multiplica- 

 tion of some species as the torrid zone to others ; but the largest 

 kinds are only found in warm latitudes. 



936. In regard to the subdivision of the Lamellibranchiata into 

 Orders and Families, great difficulty is felt by those by whom 

 this group has been most studied. By some, the presence of a 

 single or double adductor muscle has been taken as the ground 

 of the primary division ; but for the reason already stated, this 

 is unsatisfactory ( 932 ). 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 deter- 

 mine the real position of genera, which would otherwise be 

 doubtful. For the present, however, it will be desirable to adopt 

 the following arrangement into ten primary divisions, which has 

 been proposed by M. De Blainville. The names of some of 

 these, however, have been changed ; to make them correspond 

 better with those adopted by other naturalists : 



I. The OSTRACEJE, 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. 



II. The PECTIN ID5, or Pecteng (Scallop-shells) and their 

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



