168 MOLLUSCA. GASTEROPODA. 



Pliocene. Trochus, Scalaria, Turritella, Emarginula, 

 Natica, Littorina, Cerithium, Buccinum, Nassa, Purpura, 

 Fusus, Valuta. 



CLASS. SOAPHOPODA. 



The Scaphopoda include only a few genera, which in 

 some respects resemble the lamellibranchs. The body is 

 bilaterally symmetrical ; the mantle is cylindrical and 

 open at both ends, and it secretes a straight or slightly 

 curved tubular shell which is also open at both ends. 

 The foot is greatly elongated and can be protruded through 

 the larger aperture of the mantle and shell. The animal 

 is attached to the posterior part of the shell by means of 

 a muscle ; an odontophore is present, but the head is not 

 well marked, and eyes and heart are absent. The sexes 

 are separate. All the Scaphopods are marine, and they 

 often live buried in sand or mud. The earliest forms 

 occur in the Ordoviciau rocks. 



Dentalium. Shell conical or sub-cylindrical, at- 

 tenuated posteriorly, slightly curved. Anterior aperture 

 simple, not constricted ; posterior aperture smaller. Sur- 

 face smooth, annulated or ornamented with longitudinal 

 striae or ribs. Devonian to present day. 



CLASS. CEPHALOPODA. 



The Cephalopods are more highly organised than any 

 of the other mollusks ; well-known forms are the cuttle- 

 fishes, belemnites, ammonites, and the nautilus. They 

 are always bilaterally symmetrical. The head is well 

 marked, it is separated from the body by a constriction, 

 and is especially characterised by the fact that the fore- 



