MOLLUSC A. 



227 



group.) In this order there are four gills and 



A^/T\V 



c. 



Ficf. 78.— Shells of fossil forms of the Tetrabranchiate Oephalopods. The 

 exterior markings of tlie shells correspond with the position of the successive 

 air-chambers. A, Turnlites costatus, with reversed spiral. B, Ceratites nodosus, 

 C, Attji/monites planicostatua. 



numerous arms without suckers, called tentacles, and 



