SPIRULA PERONII. 27 



The most instructive way of examining the shell is to cut a thin 

 section passing through all the whorls in the plane of coiling, whereby 

 the structure of the walls and septa, as well as their mutual relation 

 is well exposed. As far as I know, such a section has not been 

 figured or described ; it is exceedingly difficult to accomplish, and 

 requires the sacrifice of many specimens. The best way to do it, 

 is to take the shell just as it is, lay it on its side on a keen, perfectly 

 flat Water-of-Ayr stone, and rub it carefully till the middle is 

 reached ; mount it then direct on the glass slip with hardened 

 balsam, and rub the other side down till the requisite thinness is 

 obtained. In putting on the cover-glass, drop some hot balsam on to 

 the slide, and do not warm it in the usual manner, as the whole thing 

 will float away in pieces. 



The walls of the shell are formed of two layers. The inner 

 consists of a semi-transparent, porcellanous material with a steep 

 imbrication, that is, it appears to be made up of layers which are 

 inclined at a good angle with the surface. It has no trace of 

 prismatic structure which would homologise it with the inner, 

 mother-of-pearl layer of the Nautilus and the Ammonites, but 

 resembles rather the outer granular layer of these though the granules 

 seem arranged in fibres which are extended at right angles to the 

 surface. Each layer readily splits away from its fellows and the shell 

 consequently is very brittle but breaks regularly in rings correspond- 

 ing with the laminse. 



The outer layer is clear and glassy, and is riddled through with 

 little tubules running parallel to the surface on which they frequently 

 open, thus giving the shell the roughness which has caused the term 

 " shagreen " to be applied to this layer, and which Sandberger (9 ) 

 homologises with the peculiar wrinkled layer found between the 

 whorls of some Ammonites, and which is similar in position to the 

 black layer of the recent Nautilus ; at any rate, it has nothing to do 

 with the shagreen on the shells of the Decapoda which consists of a 

 deposit freely poured out and hardening in spherulitic knobs^ 10 ). 



The septa are truly prismatic, and the shell-substance is identical 

 with that forming the septa of Nautili and Ammonites ; at the outer 

 periphery they expand somewhat and the successive layers of which 

 they are composed are separated, while a prolongation of the inner 

 layer of the shell-wall covers the upper surface for a short distance. 

 This latter fact, showing that the inner wall was formed after the 

 completion of the septum, proves that the septa are not morphologi- 

 cally equivalent to those of the camerated shells (Ectocochlia). On 

 the under surface there is a grey-looking deposit, constituting a half 



(9). Verstein. Ehein. Schicht. in Nassau, 1858, p. 58. 

 (10). Moynier de Yillepoix, Joum, Phys., Paris, 1892, p. 618. 



