SEMIPHYLLIDIANA. 199 



mercifully opened for the solace and instruction of his 

 reasonable creatures ? 



Family II. Semiphyllidiana. 



There dwell more curious things 

 Beneath the waters, than men ever thought, 

 Or poet feigned. 



In this family the breathing organs are arranged on the 

 right side only ; that is, only half-way round the mantle ; 

 and hence the name given by Lamarck of Les Se^niphylli- 



Their shell is smooth, sometimes calcareous, sometimes 

 horny, and either external or internal. 



The family includes two genera — Pleurohranclius and 

 UmhreUa. 



The PleurobrancJms exhibits a peculiarity in the breathing 

 organs — their position is right-sided, and their structure 

 plumose. They are occasionally conchiferous, and when 

 this occurs, the shell, being small and merely of a thin 

 horny texture, is concealed within the fleshy substance of 

 the mantle. 



The genus Umbrella has an external, dorsal, orbicular 



