BELEMNITE COMPARED WITH NAX^TILIJS. 285 



Comparing the shell of Belemnite, with that of Nautilus, 

 we find the agreement of all their most important parts to 

 be nearly complete ;* and the same analogies might be 

 traced through the other genera of chambered shells.f 



or flint, when the matter of the chalk strata was deposited upon them, in 

 a soft and fluid state. (See Allan's Paper on Belemnite, Trans. Royal 

 Soc. Edin., and Miller's Paper, Geol. Tran?. Lond. 1826, p. 53.) 



Thus of the millions of Belemnites which crowd the Secondary forma- 

 tions, only the fibro-calcareous sheatli and chambered alveoli are usually 

 preserved; whilst in certain shale beds this sheath and shell have some* 

 times entirely disappeared, and the horny or nacreous sheath or ink-bag 

 alone remain. See PI, 44", Fig. 1, 2, 3. 4, 5, 6, 7, 8. la the rare case 

 PI. 44', Fig. 7, which has aflforded the clue to this hitherto unexplained 

 enigma, we have all the three essential parts of a Belemnite preserved in 

 their respective places nearly entire. The ink-bag (c) is placed within 

 the anterior horny cup (e, e', e'';) and the chambered alveolus, (b b') 

 within the hollow cone of the posterior fibro-calcareous shell, or common 

 Belemnite. 



* The air-chambers and siphuncle are, in both these families, essentially 

 the same. 



In Belemnites, the anterior extremity of the fibro-calcareous shell, which 

 forms a hollow straight cone, surrounding the transverse plates of the 

 chambered alveolus, represents the hollow coiled up cone containing all the 

 transverse plates, which make up the alveolus of the Nautilus.. 



The anterior horny cap, or outer chamber of the Belemnite, surrounding 

 the ink-bag, and other viscera, represents the large anterior shelly chamber 

 which contains the body of the Nautilus. 



The posterior portion of the Belemnite, which is elongated backwards 

 into a fibrous pointed shaft, is a modification of the apex of the straight 

 cone of this shell, to which tiiere seems to be no equivalent in the apex 

 of the coiled-up cone of Nautilus. The cause of this peculiar addition to 

 the ordinary parts of shells, seems to rest in the peculiar uses of the shaft 

 of the Belemnite, as an internal shell, acting like the internal shell of the 

 Sepia Ofiieinalis, to support the soft parts of the animals, within the 

 bodies of which they were respectively enclosed. The fibrous structure 

 of this shaft is such as is common to many shells, and is most obvious in the 

 Pinnce. 



t Comparing the Belemnite, or internal shell of Balemno-sepia with 

 the Sepiostaire, {Blainville,) or internal shell of the Sepia Officinalis, we 

 have the following analogies. In the Sepiostaire, (PI. 44', Fig. 2, a. e. 

 and Figs. 4, 4'. 5,) the small conical apex (a) represents the apex of the 

 long calcareous posterior sheath of the Belemnite, (Fig. 7, a.,) and the 



