CLASSIFICATION. 103 



B. Decapoda calciphora. Internal shell calcareous. 



Family 11. SEPIID^. Eyes covered by skin ; littoral. Body 

 oval, with long lateral fins, uniting behind ; mantle supported by 

 cartilaginous tubercles fitting into sockets on the neck and 

 siphon ; arms with suckers, tentacular arms entirely retractile ; 

 siphon valved. Shell (cuttle-bone, sepion or sepiostaire) broad, 

 flat, thickened internally by numerous plates ; terminating behind 

 in a hollow, imperfectly chambered apex or mucro, without con- 

 necting siphon. 



Family 12. BELOSEPIID^E. (Fossil only.) Shell like Sepia, but 

 the walls of the chambers of the mucro pierced by small holes, 

 indicating the existence of a connecting siphon. Animal un- 

 known. 



Family 13. BELEMNITID^:. (Fossil only.) Animal, arms with 

 hooks. Shell a pen (pro-ostracum) attached to a chambered cone 

 (phragmocone), the partitions of which are pierced by a sub- 

 marginal, ventrally-placed siphuncle ; at the hinder end the 

 phragmocone is enveloped by a rostrum. 



Family 14. SPIRULID^. Animal, body oblong, with minute 

 terminal fins ; mantle supported by a cervical and two ventral 

 ridges and grooves ; arms with six rows of minute cups, tentacu- 

 lar arms elongated ; siphon valved. Shell spiral, whorls on the 

 same plane, not in connection, chambered ; chambers connected 

 by a ventral siphon, invested by a series of cone-shaped tubes, 

 one for each chamber. The shell is placed vertically in the end 

 of the body, and is held in place by side flaps of the mantle. 



I have adopted the above succession of families as indicating 

 a progression from the so-called naked octopods with the inter- 

 nal shell represented by cartilaginous styles, through the car- 

 tilaginous-shelled cirroteuthis, to the decapods with horny pens : 

 then those with calcareous plates and minute initial chambers, 

 the latter of which gradually become larger, are siphunculated, 

 curve, become spiral and thus form a passage into the fossil 

 tetrabranchiates and the externally shelled Nautilus. It is not 

 impossible, that, among the ancient genera, the structure of the 

 animals was such as to bridge over the gulf which now exists 

 between the two orders, and it has been recently maintained by 

 M. Munier-Chalmas, and more cautiously advanced by Dr. Paul 



