one muscular impression: Ostrea. 
ARRANGEMENT OF REPRESENTATIVE EORMS. 
365 
Order 1. Tictrabranouiata _Having four gills, many tentacles, and an external, chambered shell: Nautilus. 
^ _ , , . , . (eight arms: Octopus. 
Order 2. Dibranohiata. —Two gills; naked; ink-bag; <, 0 
(ten arms: Sepia. 
