N 



MOLLUSCA. CEPHALOPODA 257 



elongated and cylindrical ; it can be protruded through 

 the larger (anterior) aperture of the mantle and shell, and 

 serves as a digging organ. The animal is attached to 

 the posterior part of the shell by means of a muscle ; an 

 odontophore is present, but the head is rudimentary, 

 and eyes, gills, and heart are absent. The sexes are sepa- 

 rate. All the scaphopods are marine, and they usually live 

 buried in sand or mud, with only the posterior extremity 

 projecting into the water ; they range from the littoral 

 down to the abyssal zone. The earliest forms are found in 

 the Ordovician rocks. 



Dentalium. Shell conical or sub-cylindrical, tapering poste- 

 riorly, slightly curved. Anterior aperture simple, not constricted ; 

 posterior aperture smaller, without a fissure. Surface ornamented 

 with longitudinal striae or ribs. Eocene to present day. Ex. D. 

 elephantinum, Recent. Forms closely allied to Dentalium occur in 

 many Palaeozoic and Mesozoic formations. 



CLASS IV. CEPHALOPODA 



The Cephalopods are entirely marine and are more 

 highly organised than other molluscs; well-known living 

 forms are the cuttle-fishes, the squids, the paper-nautilus 

 and the pearly nautilus, whilst amongst extinct types 

 are belemnites, ammonites, and goniatites. Existing 

 forms are always bilaterally symmetrical. The head is 

 well marked and is separated from the body by a con- 

 striction ; it is especially characterised by the presence of 

 a circle of arm-like or lobe-like processes around the mouth 

 (fig. 102, e, f) ; these processes are provided either with 

 sucking-discs or with tentacles, and are used for seizing 

 food, and in locomotion. Behind the head is a muscular 

 tube termed the funnel (d), which opens in front to the 



w. p. 17 



