NAUTILOIDEA. 847 



Goniatites, and as representing an ancient type of the Ammonoids, 

 but it is now generally placed among the Nautiloids. The species 

 of the genus are Silurian and Devonian. 



Prosiphonate Nautiloids. 



All the groups of the Nautiloids which have been previously 

 considered are " retrosiphonate," having the septal necks directed 

 backwards. On the other hand, the genera Nothoceras and Bath- 

 moceras are "prosiphonate," the septal necks being directed 

 towards the aperture of the shell. The genus Bathmoceras is 

 Ordovician, and is characterised by the possession of a straight 

 cylindro-conical shell, the septate portion of which is always trun- 

 cated. The aperture is simple, and the siphuncle is marginal, and 

 is ensheathed by numerous short invaginated funnels derived from 

 the closely-set septa. In the Silurian genus Nothoceras the shell 

 is nautiloid and widely umbilicated ; the sutures are simple and 

 concave ; the siphuncle is ventral in position ; and the septal necks 

 are short and are directed forwards. 



