A, Hyatt—Brological Relations of the #urassic Ammonites. 345 
ning of the true shell or apex, with its nautilus-like septum, and 
peculiar nautilus-like umbilicus ; third, the depressed and goni- 
atite-like continuation of the form fo the shell with its accom- 
panying goniatitic septa. 
ese of course represent only their most advanced stage in 
the Ammonites proper of the Jura and Trias; they are, when 
first observed in the Silurian and Devonian, exceedingly vari- 
able in the length of the periods and other important charac- 
teristics even between the varieties of different species. They 
become invariable in the young as embryonic characteristics 
the lowest. They correspond to what naturalists are in the 
habit of calling paralled forms, often also representative forms. 
e 
Single ancestral species of the discoidal or open-whorl forms 
of the original inherited or stock form of the third stage a 
