CLASSIFICATION OF CEPHALOPODA. 343 



Classification. 



Order I. Tetrabranchiata (see preceding page). 



Family I. Nautilidae. Nautilus alone alive ; but a great series 

 of fossil forms, Orthoceras — Trochoccras. 



Family II. AmmonitidK. All extinct, but with shells well 

 preserved, so that long series can be studied. They 

 furnish striking evidence of progressive evolution in 

 definite directions. Examples — Bactrites, Ceratites, 

 BacuKtcs, Uamites, Ammonites, Heteroceras. 



Order II. Dibranchiata (see preceding page). 



Sub-Order Decapoda. Eight' shorter and two long arms. 

 Suckers stalked and strengthened by a horny ring. 

 Large eyes with a horizontal lid. Body elongated, with 

 lateral fins. Mantle margin with a cartilaginous " hook 

 and eye " arrangement. Some sort of internal ' ' shell. " 

 With calcareous internal " shell. " Spirula; Sepia; extinct 



Belemnites. 

 With horny internal ' ' shell. " 



(a) Eyes with closed cornea, e.g., Loligo. 



(b\ Eyes with open cornea, e.g., Ommastrefhes. 



Sub-Order Octopoda. Eight arms only. Suckers sessile 

 without horny ring. .Small eyes with sphincter-like 

 lid. Body short and rounded. No "hook-and-eye" 

 arrangement. No internal "shell." 

 Examples — Octopus, Eledone, Argonauta. 



