1893. OWEN. 27 
which was known to Aristotle, and had been briefly described by 
Rumphius. It was, therefore, with peculiar satisfaction that Mr. 
Owen received an example in alcohol which had been captured, ina 
dying condition, by his friend and former fellow-student at the College 
of Surgeons, Dr. George Bennett of Sydney, while cruising in the 
Polynesian seas. The results of his investigations appeared in the 
‘* Memoir on the Pearly Nautilus, with illustrations of its external 
form and internal structure,” which, to the keen regret of the author, 
was not issued from the press until three days after the death of his 
illustrious master. The animal was shown to be characterised by the 
presence of pedunculated eyes, calcareous mandibles, like a bird’s 
beak reversed, a crop, gizzard, and liver, four gills, or breathing 
organs, and by the absence of a branchial heart and ink-bag. It 
occupied the last and largest chamber of a nacro-porcellanous, many- 
chambered shell, into the last cell of which it could retract itself and 
its numerous simple non-aceptabulated arms closing the mouth of 
the shell by the dorsal arms and by a horny hood-shaped structure, 
the analogue of the aptychi or double operculum of the fossil 
Ammonite. A central siphuncular tube pierced all the septa running 
through all the chambers or so-called ‘ air-cells”” which had been 
occupied by the animal in succession. This siphuncle ‘‘ subserved a 
hydrostatic function,” enabling the animal to rise to the surface and 
sink to the bottom, possibly by.means of a gas secreted from the 
mantle. But the author’s remark that ‘‘much remains to be done 
before the theory of the chambers and siphuncle can rest on the 
sound basis of experiment and observation ”’ is still applicable.4 The 
order Tetrabranchiata was proposed to receive the recent Nautilus 
characterised by the presence of four gills, a many-chambered external 
shell, and the absence of an ink-bag; and the fossil Ammonites and 
their allies were included therein. The Dibranchiata comprised all 
the two-gilled naked Cephalopods with an internal shell, or pen, and 
an ink-bag such as the living octopoda, the squids, cuttles, and the 
fossil Belemnitide. 
In 1844 Professor Owen resumed the study of fossil cephalopoda, 
and communicated descriptions and figures of ‘certain Belemnites 
preserved with a great proportion of their soft parts in the Oxford 
Clay at Christian Malford, Wilts.” The guard sheath or rostrum, 
known as the dart or javelin, whence the name Belemnite was origi- 
nally applied, was described ; the chambered or vertically septated 
portion was named the phragmacone; while the presence of an ink- 
bag, internal shell, muscular mantle, and aceptabulated arms were 
duly noted, and the fossil molluscs were classed with the recent 
14 There appears to be no doubt that the deserted chambers of the Nautilus-shell 
contain, in the healthy living animal, a gas which serves to lessen the specific 
gravity of the whole organism... . ‘‘A certain stage is reached when no new 
chamber is formed; with regard to its origin we can only conjecture; the whole 
matter is involved in obscurity.” Ray Lankester, ninth ed., Ency. Brit., 1883. 
