302 FUNCTIONS AND INSTINCTS. 



mediate between the Zoophagan Trachelipods 

 and the Cephalopods. They have four swim- 

 ming organs. There seems a considerable 

 affinity between this tribe and the Pteropods 

 in these organs, which indicates a circular ar- 

 rangement in the univalve Molluscans. The 

 Carinaria vitrea is one of the rarest shells that 

 is known, arising probably from its extremely 

 fragile conch, which is nearly as transparent as 

 glass. A model of it in wax may be seen in 

 the British Museum. The animal is a sailor 

 like the Argonaut, to which it comes near. It is 

 found in the South Seas. There are two other 

 species known, one of which frequents the 

 Mediterranean. Some genera without shells 

 are placed in this order by Lamarck. They 

 swim horizontally like fishes, which circum- 

 stance, in conjunction with their fins or swim- 

 ming organs, induced him to place them at the 

 end of the Molluscans as near the fishes ; se- 

 veral authors consider them as belonging to the 

 Pteropods, to which they are certainly related. 



