Royal Society. 387 



to the former division, and that the Cephalopoda and Pulmonata 

 belong to the latter. 



8. Anatomical evidence shows that the Heteropoda have a haemal 

 flexure of the intestine, the Pteropoda a neural flexure ; and it is 

 almost certain that when their development is traced, the former will 

 be found to have a post-abdomen, the latter an abdomen ; there will 

 then be two great divisions of the cephalous mollusca. 



a. Those which develope an abdomen : — Cephalopoda, Pteropoda, 

 Pulmonata. 



b. Those which develope a post-abdomen : — Heteropoda, Pectini- 

 branchiata, Tectibranchiata, Nudibranchiata. 



9. Prosobranchism and Opisthobranchism may occur as secondary 

 results of either course of development. 



10. The principal nervous centres are similar in number and posi- 

 tion throughout, and differ only in their arrangement and degrees of 

 concentration. The amount of the latter does not correspond with 

 the complexity of organization of the moUusk, but rather the re- 

 verse. 



11. The organization of the vascular system is equally uniform; 

 its completeness or incompleteness is no mark of complexity or sim- 

 plicity of the rest of the organization. 



12. The cephalous Mollusca are characterized by the possession of 

 a peculiar organ, the dentigerous "tongue," whose mode of action 

 resembles that of a chain-saw. 



13. The locomotive apparatus, when fully developed, consists of 

 four parts, the propodium, mesopodium, metapodium and epipodium. 

 These parts are least modifled in such moUusks as Atlanta or Pneu- 

 modermon ; most altered and disguised in such as Cleodora or Oc- 

 topus. 



14. The term " mantle" should be confined to the surface of the 

 abdo7)ien or post-abdomen, and to the prolonged edges of that surface. 



15. It is of great importance to recollect that the " shells" are 

 probably not homologous organs in all the different forms of mollusks. 



The shells of Sepia, Spirula (?), Limax, Clausilia and Helix are 

 developed in the thickness of the mantle. 



The shells of Nautilus (?), Pectinibranchiata, &c., are developed 

 from the surface of the mantle by a quite distinct process. 



Certain curious differences appear to follow the externality or in- 

 ternality of the shell. 



An external shell in a mollusk with a haemal flexure, e. g. Atlanta, 

 has its columellar axis below the ajierture. 



An external shell in a mollusk with a neural flexure, e. g. Nautilus, 

 has its columellar axis above the aperture. 



An internal shell in a mollusk with a neural flexure, has its colu- 

 mellar axis below the aperture, e. g. Spirula, Clausilia, Helix. 



In the course of the memoir the author incidentally introduces a 

 number of new, and, as he believes, important facts, with regard to 

 the nervous, circulatory and urinary systems ; and describes at 

 length the mechanism of the " tongue" and an organ similar to the 

 " crystalline style" of bivalves, found in the Strombidae. 

 2C2 



