GEOLOGICAL DIST.RMJI TinX AM) DEVELOPMENT. 47 



posed elevations of the periphery, on each side of the mantle. 

 Between these and the edge of the mantle, two longitudinal 

 ridges mark the rudiments of the epipodia, while the mouth 

 appears in the middle line in front of the mantle, and the anus,, 

 with the rudiments of the gills, behind it. The rest of the 

 blastoderm forms the Avails of a vitelline sack, enclosing the 

 larger blastomeres. 



" The pallial surface now gradually becomes more and more 

 convex, the posterior margin of the mantle growing into a free 

 fold, which encloses the pallial chamber and covers over the gills. 

 " The internal shell is developed iu a sac formed by an involu- 

 tion of the ectoderm of the mantle. The epipodia unite behind, 

 and give rise to the funnel, while the antero-lateral portions of 

 the foot grow over the mouth, and thus gradually force the latter 

 to take up a position in the centre of the neural face, instead of 

 in front of it. The yolk sack gradually diminishes, and the con- 

 tained blastomeres are finally taken into the interior of the 

 visceral sac, into which the alimentary canal is gradually drawn." 

 HUXLEY, "Anatomy of luvertebrated Animals." 



Such is the language of a master equally of the art of conden- 

 sation of language and of anatomy, and who has enriched 

 malacological anatomy impartially with that of other branches of 

 natural science by his researches. 



At a later period of development, respiratory movements are 

 performed by the alternate dilatation and contraction of the 

 mantle, and the position of the ink-bag is revealed by its color 

 through the transparent llesh. The shell of the young Sepia, 

 observes Kolliker, by the time it is prepared to leave the nidi- 

 mental capsule has already formed, but except the nucleus, which 

 is calcified, its line layers are horny and transparent. The (ins 

 are proportionally broader than those of the mature animal. 

 The development of Nautilus, equally with its nidus is unknown. 



Geological Dixlrihnti.tni, and Development. 



hi all fossiliferous strata the cephalopods hold a high position 

 by reason of their large development in species and individuals. 

 Those species possessing siphunculatecl shells of solid texture 



