l- s K.\ I KK.NAI, s 



tin- c\cs a iv lame and prominent : the yolk-ha^. or vitellus, is 

 next seen very distinctly. Mild the processes extending from the 

 head are more elongated. Here, however, I was obliged to stop. 

 this being tlir most perfectly developed embryo 1 could find 

 amongst the ova. The eggfi in contael \vilh the front part of the 

 body-whorl of the shell, where the egg-mass is attached by the 

 ulntinous tlireads. are the most forward in their development. 

 while those in the posterior part of the chamber are much le-^ 

 matured. AKTIII K APAMS, /ool. Yoy. Sa ma rang. f. lxf>0. 



Tlu 1 niultiloenlar external shells ( Nautilus. A mmon'n- 

 distinguish an order of cephalopoda breathing by four instead ot 

 two branehia'. and with the arms much reduced in si/e and sub- 

 divided into tentacles. The shells are capable of containing tin- 

 entire animal in the cavity above the last aerial chamber, to the 

 wall of which it adheres by two strong muscles. These shells 

 are composed of two layers, the external or porcellanous con- 

 taining the colors, and the internal, which is pearly, and which 

 includes the partitions or septa'. These septa 1 , which are straight 

 or arcuated in Nautilus, in Ort .hoccrat it es, etc.. are angulaled in 

 (ioniatites. and with infinitely ramilied lobes in Ammonites, 

 Hamites, Turrilites. and oilier fossil genera. 



The inner pearly layer of the shell, as well as the septa, is 

 formed by the body of the animal, whilst the outer porcellaiious 

 layer is constructed by the mantle-ma ruin. There is additionally 

 deposited, on the spire side of the Nautilus shell, a third thin. 

 black, grainy layer, which can be readily scraped oil'. This sub- 

 stance can be detected also in many fossil tet rabranchiate-. 



Saudberger finds the hardness of the porcellanous layer of 

 Nautilus, I-.") to ." ; the nacreous layer. .'K> to !; whilst the 

 specific gravity of the former is L'-i;i;f>. and of the latter. 1 '.V.M'i. 



The structure of the shells of existing testaceous cephalopods 

 is. on the whole, more analogous t,o that of bivalves than to that 

 of the irastrropods. the three layers of perpendicular lamiim-. so 

 characteristic of the latter, beini; here quite indistinguishable. 

 The shell of Nautilus is the only one in which the presence of 

 //iv* layers i^ obvious, from their difference of texture. A thin 



SeCtieD of Ihe external la\er of the shell of \unfilu.< /'/i//;y;/7///S. 

 taken parallel to the surface, shows that it is made up of an 

 aggregation of cells of various sizes, those strata which are 



