AGE OF MOLLUSKS. 



197 



1. Lingula. 2. Rhynconella. 3. Pentamerns. 4. Strophomena. 5. Spirifer. 

 _6. Murchisonia. 7. Orthoceras. 8. Lituites. 9. Maclurea. 

 Fig. 106. 



which have arms or feet about the mouth, the name com- 

 ing from the Greek works kephale, head, and pous, foot. 

 The shells of this class are more often curved on one 

 plane, as 8, than spiral, as 6. In 8 we have a chambered 

 shell an arrangement that is found in almost all of this 

 class the cavity being divided by partitions into cham- 

 bers, which have a tube, called the siphuncle, running 

 through them. The use of this is explained in my Nat- 

 ural History, 554. 2. Gasteropoda. The name of this 

 class comes from gaster, belly, and pous. These mollusks 

 creep on a broad disk or foot, as you see represented in 

 Fig. 107 (p. 198). The head and the broad foot, which 

 are out of the shell as the animal crawls along, can be 

 withdrawn within the shell. The common snail is an 

 example of this class. 3. The Acephals. These have no 

 heads, as the name indicates, the a being the a privative 

 of the Greek. Of these there are two divisions the com- 

 mon bivalves, as oysters and clams, which have two equal 

 valves, and those which have two unequal valves. These 



