OF CENTRAL CANADA PART IV. 



275 



B. MOLLUSCA VERA. 



The mollusca proper, as distinguished from the molluscoidea, 

 possess distinct branchiae, or, in default of these, a pulmonary sac ; 

 and they have also, typically, a systematic heart. In most, the body 

 is enclosed in a calcareous shell, as in the oyster, whelk, &c.; but in 

 some forms the shell is internal, and in others it is absent or rudi- 

 mentary. They fall primarily under two sections : Aglossata, in 

 which the mollusk has no " lingual ribbon " or odontophore ; and 

 Glossophora or Odontophora, in which the animal is provided with a 

 long, narrow " tongue " or " radula " thickly set with rows of minute, 

 rasping teeth, of variable form and mode of arrangement in different 

 genera. The Aglossata comprise a single class, Lamellibranchiata. 

 The Glossophora are sub-divided into the following classes : Scapho. 

 poda ; Pteropoda ; Gasteropoda ; Heteropoda ; and Cephalopoda. 



CLASS 1. LAMMELLIBRANCHIATA. ( = CONCHIFERA, PELECYPODA, &C.) 



The lamellibranchs are entirely aquatic, and mostly marine types. 

 They secrete an external, calcareous shell composed of two separate 

 valves, united, in most forms, by interlocking points or teeth, and by 

 a narrow elastic ligament. All are headless. They are commonly 

 classed under two leading sections : Asiphonida and Siphonida. In 

 the latter the animal possesses a pair of long or short siphonal tubes, 

 capable, more or less, of extension beyond the shell. One of these 

 tubes serves for the ingress of water, and the other for the expulsion 

 of this after the air has been extracted from it, as well as for the 

 removal of effete matters generally. The Asiphonida are without 

 these siphonal tubes, although there is an approach towards them in 

 one of their families, the Unionidce. These sections are again sub- 

 divided into two groups or Orders, as in the following arrangement : 



, . , ( 1. Pleuroconcha. 

 Asphonida 1 



I 2. Orthoconcha. 



Siphonida 



( 4. Smupalhata. 



1. Pleuroconcha : In this subdivision, the valves of the shell are 

 mostly of unequal size ; and, in normal position, one valve is the 

 under, and the other the upper valve. In the interior of each valve 

 there is one large muscular impression, or one large and one 



