E. S. L. Sc/marz — Shell- Structure of Ammonites. 253 



The outer layer then, though it can be demonstrated in a few cases, 

 is ordinarily absent in the preserved shells. 



The septa of Ammonites are identical in structure with that of 

 the nacreous layer, though the septa of the recent Nautilus presents 

 many differences from the true nacre. 



EXPLANATION OF FIGURES. 



Fig. 1. Section of the extreme edge of the living -chamher of Am. (Microceras) 

 planicosta, Sow. a, so-called outer layer passing between portions of 

 the prismatic layer at 6 ; c, prisms slightly bent from the straight line ; 



d, layer separating the prismatic from the vanished outer layer ; 



e, terminal rim of conchiolin. 



Fig. 2. Section of the same showing the so-called outer layer running between 

 portions of the prismatic layer. 



