in Cephalopoda {Siphonopoda) . 309 



resembles the Belemnite-guard. Here too, then, the sign of 

 lineage is to be found in the young shell. 



Conclusion. 



The views here put forward — suggested bj the macroscopic 

 and microscopic structure of the Sepia-owixwiA and shell — are 

 confirmed by comparison with other shells ; explained by the 

 life of individuals, they illustrate the history of the race. The 

 views are these : — The whole of the true shell and the whole 

 of the sheath are formed first in chitinous membranes, secreted 

 by the visceral hump and by the mantle respectively ; these 

 become calcified by the deposition in their interstices of arra- 

 gonite and calcite respectively ; there is no intussusception, 

 except of lime, and that is probably a physical process. Se- 

 cretion of chitin continues after growth ceases^ and may be 

 accelerated in phylogeny. The rate at which lime is depo- 

 sited is independent of the animal ; hence extent of calcifica- 

 tion varies inversely as rapidity of chitin secretion. 



In obedience to these principles and to others long since 

 established the results which we see have been and are being 

 produced by agencies already known. 



Papeks and Works referred to. 



1. \V. B. Cabpentek. — '' Ou the Microscopic Structure of Shells,"' 



14tli Rep. Brit. Assoc, York, p. 1 (1844). 



2. Edwards and Wood. — " The Eocene Cephalopoda and Univalves,'' 



Pala3ontogr. Soc. 1848-77. 



3. S. P. Woodward. — ' Manual of the Mollusca,' ed. 4, 1880. 



4. P. Fischer. — ' Manuel de Conchyliologie,' &c., 1883. 



5. Hyatt. — "Fossil Cephalopoda in the Museum of Cornp. Zool. ;" Proc. 



Amer. Assoc, xxxii. Minneapolis, p. 323 (1883). 



6. V. G UMBEL. — "Ueber die Beschaffenlieit der Mollusken-Schalen," 



Zeitschr. deutsch. geol. GeseUsch, xxxvi. p. 38G (1884). 



7. F. MiJLLEB. — " Schalenbildung bei Lamellibranchiaten," Zool. 



Beitrage, Breslau, p. 206 (1885). 



8. RiEFSTAHL. — " Die Sepien-Scbale u. ihre Beziehuugen zu den 



Belemniteu," Palaeontographica, Bd. xxxii., 1886. 



9. Ajs^on. — " Bemerkungen iiber Cephalopoden-Schalen," Der Natur- 



forscber, Jabrg. xx. No. 18, p. 153 (April 30, 1887). 



10. Bather.— "The Growth of Cephalopod Shells," Geol. Mag. 



dec. iii. vol. iv. p. 446 (October 1887), 



