one muscular impression: Ostrea. 
ARRANGEMENT OF REPRESENTATIVE FORMS. 
365 
b I 
- £ 
« 63 
5. $ 
a & 
H S 
* -a 
- § 
bfl 
*5 'o 
I a 
5 *3 
^ S3 
•§ ^3 
It 2 
8 | 
1 
« -S 
•o 5 
s 
«*> c 
o 
i 8 
3 13 
o 3 
O. C3 
o 2 
© O 
■p > 
1/3 w 
Cj 
o 
3 g Jo 
2 2 
c3 « 
*§ 1 
C Q) 
a a 
«3 2 
o-S 
© 
o 
Order 1. Tetbabbanchiata.— Having four gills, many tentacles, and an external, chambered shell: Nautilus. 
Order 2. Dibbanohiata.— Two gills; naked; ink-bag; \ f lght arms 0cto P m ‘ 
(ten arms: Septa. 
