436 Geological Society. 



2. A later group, with unpaired tubercles in the involute stage. 

 These genera are most readily distinguished by sutural characters, 

 namely, the relative depths of the external lobe (EL) and the first 

 lateral lobe (IL), and by the width of the external saddle (ES). 



(a) With narrow ES (not reaching to the outer tubercles). 



Liparoceras Hyatt. IL and EL about equal in depth. Genolectotype, 



Ammonites striatus Bronn. 

 Becheiceras, gen. nov. IL deeper than EL. Genoholotype, Ammonites 



bechei "Wright. ('Lias Ammonites ' pi. xli, fig. 1.) 

 Anisoloboceras, gen. nov. IL much deeper than EL, the ventral lobules 



of IL almost meeting under EL. Genoholotype, Ammonites nautili- 



formis J. Buekman. 



(b) With wide ES, reaching to the outer tubercles. 



JEgoceras "Waagen. EL and IL about equal in depth, IL symmetrical. 

 Genolectotype, Ammonites planicosta d'Orbigny. 



Androgynoceras Hyatt. IL and EL about equal in depth, IL asym- 

 metrical. Genolectotype, Ammonites hybrida d'Orbigny. 



Oistoceras S. S. Buekman. Ribs with sharp peripheral curve. Suture 

 similar to Androgynoceras. Genoholotype, Ammonites figulinus 

 Simpson. 



Amblycoceras Hyatt. Ribs with slight peripheral curve. IL shallower 

 than EL. Genoholotype, A. capricornus Hyatt, 1900. 



These ammonites generally occur in the upper part of the Lower 

 Lias, where it has been usual to recognize a capricornus zone over- 

 lying a striatus zone. Careful collecting has shown, however, that 

 there are several horizons with Capricorn ammonites of different 

 series and several with the involute forms evolved from them, as 

 shown below : — 



... f Bituberculate ammonites of the A. nautiliformis series. 



" M ZOne ' 1 Bituberculate „ „ Oistoceras. 



f Capricorn „ ,, Oistoceras. 



Bituberculate „ „ the A. bechei series. 



Bituberculate „ „ JEgoceras & Androgynoceras. 



Bituberculate „ „ Amblycoceras. 



I Capricorn „ ,, Amblycocei'as. 



(^Capricorn „ „ JEgoceras, Androgynoceras. 



f Capricorn „ „ Beaniceras. 



J Bituberculate „ „ Liparoceras. 



ibex zone ] Capricorn „ „ Liparoceras. 



L Bituberculate „ „ the first group (with paired 



tubercles). 



In no locality that has been examined is the complete sequence 

 shown. The absence of some groups is due to the original distri- 

 bution of the ammonites ; in other cases it is due to non-sequences 

 (for example, the upper part of the davoei zone is not represented in 

 Gloucestershire). 



Two groups of Lias Ammonites are recognized, namely: (i) 

 those which were evolved directly from a globose ancestor ; this 

 includes the Liparoceratidse, Echioceratidse, Hildoceratidae, Poly- 

 morphidse, Deroceratidse ; and (ii) those which passed through an 

 intermediate broad- ventered (cadicone) stage; these include the 

 Amaltheidae and Dactyloidse (with Beaniceras). 



