284 Reports & Proceedings — Geological Society of London. 



action of the gases occluded in the metal and condensed into the 

 centre on quick cooling. 



A list of about one-half of the experiments is given in the 

 Appendix. 



II. — Geological Society of London. 



A^prtl 17, 1918.— -G. W. Lamplugh, F.R.S., President, in the 



Chair. 



The following communication was read : — 



" The Evolution of the Liparoceratidse." By Arthur Elijah 

 Trueman, M.Sc, E.G.S. 



The Ammonites considered include several sub-parallel series, of 

 which four genera were indicated by Mr. S. S. Buckman in Yorkshire 

 Type Ammonites. The details of ontogeny and the sutures, which 

 had not hitherto been compared, have been employed in constructing 

 tables showing both the biological and the stratigraphical relations 

 of the various species ; a revision of the existing classification is 

 proposed. 



The early members of each series are similar " Capricorn" forms 

 with slender whorls and stout ribs (for instance, A. capricornus, 

 A. latcecosta, A. maculaius). In somewhat later examples the 

 outer whorl is swollen, and has paired tubercles (for instance, 

 A. heterogenes). From this stage the tendency is. to shorten the 

 period with slender Capricorn whorls by accelerating the development 

 of bituberculation and prolonging the period of pre-costate globose 

 whorls; thus the most advanced members of each series are stout 

 bituberculate forms (for instance, A. striatus, A. lecTiei), which do 

 not pass in development through a Capricorn stage. 



The following genera maybe recognized; each includes ammonites 

 of the three types mentioned above : — 



1. An earlier group, with tubercles paired in the involute stages ; 

 Radstock (Somerset) is the only British locality where these 

 ammonites have been found. 



Parinodiceras, gen. nov. Elevated whorl, paired tubercles, the inner and 

 outer rows widely separated. Genoholotype. Ammonites striatus 

 parinodus, Quenstedt (1884, pi. xxviii, fig. 6). 



Gen. nov. Round whorl, with the rows of tubercles placed close together. 

 Genoholotype, a specimen to be figured as a new species. 



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 (XL), and by the width of the external saddle (ES). 



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



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



A^nmonites striatus, Bronn. 

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



bechei, Wright. {Lias Ammonites, pi. xli, fig. 1.) 

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



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



formis, J. Buckman. 



