LAMELLIBRANCHIATA. 683 



(Tooth-shells). The shell in the Encephalous Molluscs is of a 

 very different nature in different cases, but all the members of this 

 section possess a complicated series of lingual teeth, and they have 

 for this reason been grouped together by Professor Huxley under 

 the name of Odontophora. 



As regards their distribution in space, all the members of the 

 Mollusca are aquatic in their habits with the exception of a portion 

 of the Pulmonate Gastropods, many of these, however, being in- 

 habitants of water, though others are strictly terrestrial. The 

 water-inhabiting Molluscs may live in the sea, or in brackish or 

 fresh waters. Owing to their generally living in water, and 

 owing also to their so commonly possessing hard structures, whether 

 external or internal, no fossils are more abundant or more import- 

 ant than the remains of the Mollusca. As regards the general dis- 

 tribution in time of the Molluscs, all the principal classes (viz., the 

 Lamellibranchiata, Gastropoda, and Cephalopoda) are represented 

 in the Upper Cambrian deposits, and it is therefore clear that we 

 have at present no knowledge of the really primordial types of the 

 sub-kingdom. Speaking generally, the chief representatives of the 

 Mollusca in Palaeozoic time are the chambered Cephalopods (Tetra- 

 branchiatd), the Dibranchiate Cephalopods apparently not having 

 come into existence, while the Lamellibranchs and Gastropods 

 show a comparatively limited development. In Mesozoic time, 

 the Dibranchiate Cephalopods make their first appearance, and 

 undergo a vast development, while the Tetrabranchiate division of 

 this class has also a wonderful representation. The Lamelli- 

 branchs are very largely represented in the Mesozoic deposits, but 

 the Gastropods still play a subordinate part. In Kainozoic time, 

 on the other hand, the Cephalopods undergo an extraordinary re- 

 duction, the group of the Tetrabranchiates becoming almost extinct, 

 while the Lamellibranchs and Gastropods, the latter particularly, 

 assume a predominant position. At the present day, the Lamelli- 

 branchs and Gastropods are the two leading classes of Molluscs, 

 and both seem to have attained their culminating point in existing 

 seas. 



Class I. Lamellibranchiata. 



The Lamellibranchiata ( = Conchifera or Pelecypoda) are often 

 familiarly spoken of as the " Bivalve Molluscs," and are characterised 

 by the absence of a distinctly differentiated head, and by having the 

 mantle divided into two lobes, right and left, each of which secretes a 

 shelly investment ^ so that the body is more or less completely enclosed 

 in a bivalve shell. There are 07ie or two pairs of lamellar gills on 

 each side of the body. An odontophore is zv anting. The sexes are 

 distinct or united. 



