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 Spirals ; 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 spirilla, 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 Kumphius, 

 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 (X. 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 Horloles, Montf.) — Others remaining straight throughout their 

 growth are the Ort/ioceratites. 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 theinnercone, or what has been called theah eolae. 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 considere'es zooloffiquement et geohgigue- 

 ment, by Blainvillc, Paris, 1827 ; and that <>f M. I. S. Miller (in the same 

 subject, in vol. ii. part 1, of the Geological Trans., I.oml., 1826. [The 

 English student "ill find the fullest details in Bucklmd's Bridgewater 

 Treatise.] M. de BlainviQe 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 lias, or Ins not, a small 







I — llrlcuimlci. 



* The itructure of thia singular Ccphalopod bn been fully described 

 and lUutimMd ll • vrrv admirable manner, by Mr. Omcii, iu bit 

 " aftfMolrOa t\r Vfiirly X'tutilitl," 1.(111(1,, lKl'.'. — Kn. 



t It i iiy Ki\e the student an Idea of the nature of the evidence on 



which fuitilt arc occasionally referred m In ini; ti|> r, that 



Kaspail bellcvra the Uclcinnilea to he Ibl cutaneous append* i 



ionic »ea annua], perhaps allied to the Scaurehiua, (, Kchiuiu).— Ki». 



