42 THE LIFE OF THE MOLLUSCA 
of Ammonites and their kindred, that from the close 
parallelism of their shells, by which alone they are 
known, were probably similar in structure to the 
Nautiloidea, and are therefore better classed with 
them than in a group apart. 
Order IJ.: Drprancuia (Plate XXV.), or those 
having only two gills, and the shell generally more 
or less internal, The funnel is a complete tube, and 
the eyes have a crystalline lens. 
Sub-Order 1: Decapopa, or those with ten arms. 
Two of these, the “tentacular arms,” situated on 
each side between the third and the last pair, are 
more or less retractile into special pouches, and as 
a rule only bear suckers at their free extremities. 
The eight ordinary arms are shorter than the body. 
There is generally a fairly well-developed internal 
shell, and usually lateral fins. 
Two tribes are distinguishable: (2) Those (Oigop- 
sida) in which the cornea covering the eye is incom- 
plete—z.e., has an aperture left. This includes Sfirula, 
many of the Pen-and-Ink Fishes, and probably 
such fossil forms as the Belemnites, Belemnoteuths, 
etc. (6) Those (Myopsida) in which the cornea is 
complete, like the Common Cuttle (Sepia), the 
Common Pen-and-Ink Fish (Loligo), and their fossil 
kindred. 
Sub-Order 2: OcTopopa, or those with eight 
similar arms all longer than the body. 
These again fall into two tribes: (a) LEIOGLossA, 
