224 



AN INTRODUCTION TO ZOOLOGY. 



spikes and spines, which serve at once for ornament 

 and defence; a striking instance is the Venus' Comb, 

 Murex tenuispina. 



Kg. 77.— Shells of Siplionostonmtous Gasteropods. 1, Venus' Cnmb ITui-cr 

 Unuispina; 2, Sinistral vaviety nf the Spindle-shell, i'lisiw contra ri us ■' 3 The 

 small Whelk or Purple, Purpura turtUus. , ' ' 



The spiral of the shell is occasionally found to be 

 reversed, and in some genera it is always so. These 



