SYSTEMATIC I5 



platycones of the Upper Lias Hildoceratoids, it is advisable to note 

 this character. It presents itself in three main shapes, which may be 

 diagrammatically represented as follow : — 



x ' • 



123 

 the whorl in each case lying to the right, so that the upper part represents 

 contact with the surface of the outer whorl, and the lower part contact 

 with that of the inner whorl. Therefore No. 1 may be called 'undercut,' 

 No. 2 upright, and No. 3 sloping. But there are further possible 

 modifications : each one may be plain as illustrated, concave or convex, 

 making, therefore, nine possible shapes. And, of course, there might be 

 further modification of these, like subconcave or perconcave. 



The following is an analysis of some Upper Lias genera : — 



Undercut Harpoceras, Tardarpoceras 



Upright Phaularpites 



Upright convex Paltarpites, Tiltoniceras 



Sloping Hildoceratoides, Orthildaites 



Sloping concave Eleganticeras, Elegantuliceras 



Hildoceras, Pseudolioceras 



Sloping subconcave Harpoceratoides 



Much sloping, subconcave Ovaticeras 

 Sloping passing to convex Hildaites. H. subserpentinus passes to sloping 



convex. 

 Sloping convex Murleyiceras. In adult M. gyrale concavity 



appears. 

 Sloping subconvex Glyptarpites 



These inner-marginal characters are not so distinct in young forms, 

 and in ontogenetically or phylogenetically gerontic forms there may be 

 alteration, decline, or exaggeration. 



In the Inferior Oolite there are, roughly, two main divisions : the 

 Ludwigoids have a sloping, often strongly concave inner margin, while 

 the Sonninines, like Witchelloids, have a more or less upright inner 

 margin, sometimes convex. 



In some cases the junction-line of inner margin and inner area of 

 whorl is shown rather distinctly, as a narrow longitudinal ridge. In 

 some other cases the inner margin is quite undefined — the inner area of 

 the outer whorl falling so gradually towards the plane of the overlapped 

 whorl that no inner area is defined. 



