DESCENT OF THE OCTOPODA. 89 



shaped to a brevicone Ortlwceras, round which is deposited a strong 

 calcareous " guard " prolonged backwards as a cylindrical rod and 

 enclosing and preserving the naturally caducous protoconch (hence 

 Bather's suggested name " Coleoiclea " for the Belemnites and their 

 derivatives).! 11 ) In front, the sheath of the phragmacone rises up to 

 form the thin pro-ostracum, which probably held up the mantle. 

 The Belemnoidea commence in the upper Trias of the Alps, and 

 cease in the top of the Chalk, attaining their maximum in the Lias. 



Representatives of the Sepiadse and Teuthidse are first met with 

 in the Lias, but owing to the fragility of their shells, they are only 

 found under exceptional circumstances. The shell of the ordinary 

 Cuttle-fish is possibly a modification of the Belemnite structure! 12 ) ; 

 the phragmacone becomes hypertrophiecl, and instead of a succession 

 of well marked septa there arises a spongy tissue of innumerable fine 

 calcareous lamellae ; the pro-ostracum remains as before ; and the 

 guard dwindles away to a small peak. In Loligo, all calcareous 

 matter has disappeared from the shell, and only a chitinous structure 

 occurs. Then, till recently, the shell sac was supposed to abort 

 completely, and the Octopoda arose without any calcareous skeleton 

 whatever. 



Everything would fit in nicely if the female of one of the 

 Octopoda, viz., Argonaut a, the paper Nautilus, had not an external 

 shell. This structure was regarded merely as an accident ; was 

 formed by the arms and not by the body at all ; and was considered 

 in no way homologous with the external shells of the Ammonites and 

 Nautili, because it was not camerated. But Hyatt! 13 ) has shown 

 that half of it is formed by the body-mantle ; Suss! 14 ) and Stein- 

 mann! 15 ) have shown that it closely resembles some of the Cretaceous 

 Scaphites in outer shape ; while the consideration of how the 

 chambers of the latter are formed, shows that the absence of these 

 is no essential at all. For the shell of all Ammonites and Nautili is 

 formed of an outer porcellanous or granular layer, and an inner 

 nacreous or prismatic one, the latter also forming the septa. To 

 produce this nacre, it appears necessary that the mantle should be 

 wholly occupied with that function ; but in the Argonaut the mantle 

 surface bears chromatophores, and hence is used for other purposes 

 than secreting the shell ; therefore, all the parts represented in the 



(11). Ann. Mag. Nat. Hist., 1888, p. 302. 



(12). Voltz, Mem. Soc. Hist. Nat. dc Strassbourg, Vol. I, p. 1, 1830; see also 

 P. A. Bather, loc. cit. 



(13). Embryology, Bull. Mm. Comp. Zool, Vol. III. 



(14). Ueber Amnion., Sits. ber. dcr Wien Akad., Bd. LII, p. 71, 1866 ; ib. LXI, 

 p. 305, 1870. 



(15). Vorlauf. Mittbeil. u. cl. organiz. cler Amnion., Bcr. Naturforscher Qeselh, 

 Freiburg, Vol. IV, 1889. 



