III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 89 



ing the flying membrane, but also in the proportions of 

 different parts of the spinal column and the hinder (pelvic) 

 limbs. 



Bivalve shell-fish (i.e. creatures of the mussel, cockle, 

 arid oyster class, which receive their name from the body 

 being protected by a double shell, one valve of which is 

 placed on each side) have their two shells united by one or 

 two powerful muscles, which pass directly across from one 

 shell to the other, and which are termed "adductor muscles," 

 because by their contraction they bring together the valves 

 and so close the shell. Now there are certain animals which 

 belong to the crab and lobster class (Crustacea) a class 



CYTHERIDEA TOROSA. 



An ostracod (Crustacean), externally like a bivalve shell-fish (Lamellibraneh). 



constructed on a different type from that on which 

 the bivalve shell-fish are constructed which present a 

 very curious approximation to both the form, and, in a 

 certain respect, the structure of true bivalves. Eeference 

 is here made to certain small Crustacea certain phyllo- 

 pods and ostracods which have the hard outer coat of 



