CEPHALOPODES. 
341 
that the successive layers, instead of remaining parallel and in nigh approximation, were to become 
concave towards the body, more distant, each growing a little in breadth, and making an angle 
between them, we should then have a very elongated cone, rolled up spirally on one plane, and divided 
transversely into chambers. Such is the shell of Spirula ; which has these additional characters, that 
the turns of the spire do not touch, and that a single hollow column, occupying the interior side of 
each chamber, continues its tube with those of the other columns even to the extremity of the shell. 
This is what is named the Syphon. 
Only one species {Nautilus spirula, Linn.) is known. 
The shell of the Nautilus, properly so called, differs from that of the Spirula in this,— that the septa increase 
very rapidly, and that the last turns of the spire not only touch, but envelope the preceding. The syphon is in the 
centre of each partition. The common species {Nautilus pompilius, Lin.) is very large, silvered within, and 
covered externally with a whitish crust, varied with reddish somewhat undulated bands. According to Rumphius, 
its animal should be in part lodged in the last cell, and should have the sac, the eyes, the parrot-like beak and the 
funnel of other Cephalopods ; but its mouth, instead of their great feet and arms, should be surrounded with 
several circles of numerous little tentacula, destitute of suckers. A ligament springing from the beak should run 
through the syphon, and fix the animal to it. It is probable also that the epidermis is prolonged over the exte- 
rior of the shell ; but we may conjecture that it is thin upon such parts as are vividly coloured.* 
We meet with specimens of Nautilus {N. pompilius, B. Gm. List. 552 ; Ammonia, Montf. 74), in which the last 
whorl does not envelope nor conceal the others, but in which all the whorls, although they touch, are visible,— a 
character which approximates them to the Ammonites ; yet in every other respect they so closely resemble the 
common species that it is difficult to believe they are not a variety of it. 
Among fossils there are Nautili of large and moderate sizes, and of figures more varied than now exist in the ocean. 
We also find among fossils certain chambered shells, with simple septa and a syphon, in which the body is at 
first arched, or even spiral, but the last-formed parts of it are straight : these are the Lituus of Breyn, in which 
the whorls are either contiguous or separate, (the Hortoles, Montf.)— Others remaining straight throughout their 
growth are the Orthoceratites. It is not improbable that their animals had some resemblance to that of the 
Nautilus, or to that of the Spirula. 
The Belemnites 
Belong, probably, to the same family, but it is impossible to be sure of this, since they are only found 
in a fossil condition. Their whole structure, however, shows that they were internal shells.f They 
have a thin and double shell, that is to say, composed of two cones, united 
at their base, and the interior of which, much shorter than the other, is itself 
divided internally into chambers by parallel septa, concave on the side that 
looks to the base. A syphon extends from the summit of the exterior cone 
to that of the internal cone, and is continued hence, sometimes along the margin 
of the septa, and sometimes through their centre. The space between the 
two testaceous cones is filled with a solid substance, composed either of ra- 
diating fibres or of conical layers, which envelope each other, and each of 
which rests on the margin of one of the septa of the inner cone. Sometimes 
we find only this solid part ; at other times we find also the nuclei of the cham- 
bers of the inner cone, or what has been called the alyeolae. Oftener these nuclei, 
and even the chambers, have left no other traces behind than some projecting 
circles within the inner cone ; and in other instances, the alveolae are found 
in greater or less numbers, and still piled or strung together, but detached 
from the double conical case which had inclosed them. 
The Belemnites are amongst the most abundant of fossils, particularly in 
beds of chalk and compact limestone. The most complete works upon them 
are the Memoire sur les Belemnites considerees zoologiquement et geologique- 
ment, hy Blainville, Paris, 1827 ; and that of M. I. S. Miller on the same 
subject, in vol. ii. part 1, of the Geological Trans., Lond., 1826. [The 
English student will find the fullest details in Buckland’s Bridgewater 
Treatise.] M. de Blainville distributes them from characters derived from 
the greater or less depth to which the inner cone, or chambered part, pene- 
trates ; from the margins of the external cone, which has, or has not, a small tit:- los.— Ueienmites. 
♦ The structure of this singular Cephalopod has been fully described 
and illustrated in a very admirable manner, by Mr. Owen, in his 
“ Memoir on the Pearly Nautilus,” Lund., 1832. — En. 
t It may give the student an idea of the nature of the evidence on 
which fossils are occasionally referred to living types, to mention that 
Raspail believes the Belemnites to be the cutaneous appendages of 
some sea animal, perhaps allied to the Sea-urchins, (^Echinus). — Eu. 
