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 II. : Dibranchia (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 i : Decapoda, 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 : (a) Those (Oigop- 

 sida) in which the cornea covering the eye is incom- 

 plete— i.e., has an aperture left. This includes Spirula, 

 many of the Pen-and-ink Fishes, and probably 

 such fossil forms as the Belemnites, Belemnoteuthis, 

 etc. {b) 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 : Octopoda, or those with eight 

 similar arms all longer than the body. 



These again fall into two tribes : (a) Leioglossa, 



